Apart from subsaharan Africa and South America, I have visited most regions of the world.
Weather-wise, mediterranean regions like California, the South-East and South-West of Australia and of course the Mediterranean basin are the best. Scandinavia and Eastern Europe get too cold in winter. Ireland, Britain, the Benelux and northern France have a remarkable mild climate in winter for their latitude (thanks to the Gulf stream). I closely observed the weather forecast in Europe this winter, and was surprised to see than London and southern England were almost always warmer than northern Greece and Turkey !
Economically, it depends what kind of job your are up to. As a self-employed or exercising a liberal profession (lawyer, doctor...), I'd say that all the high GDP countries are good. There are variant for some jobs. Medical doctors are better paid in Belgium than in France, and better in France than in the UK. But in finance, it may be the other way round. For employees, especially if you want to be able to change job easily, countries with a low unemployment rate are better (e.g. north of Europe, US, Japan). The US has a particularily flexible job market, but that means also less job security, and lower wages for low-level employees (e.g. in supermarkets). Germany, France and Italy all have high (official) unemployment rates, but (or maybe "because of") good social benefits for the unemployed. Australia and Canada are pretty good economically.
For the people, it's obviously too personal to give an opinion. It highly depends on who we meet. For the comfort of society in general, the UK is on top of my list because of the courtesy (it still depends a lot on what kind of people one frequents). Belgium is very good too (I'd say that people more courteous and helpful than in Japan), as long as we do not deal with the lower classes. Italy and France can be tiring and irritating (esp. in big cities). Spain and Eastern Europe are at the bottom of my list for Europe (too many people trying to cheat you as a foreigner, and less friendly or helpful than average).
As for the culture and history, France, Italy and the UK top the list. They have plenty of museums, sights, and cultural life (music, theatre...). Belgium is close behind. Germany is good for classical music. Some cities are much better than the national average in each country. Australia and NZ are surely the poorest developed countries in terms of history, and classical cultural events. In the US, the East coast is stronger than the rest. However, big cities in North America are also very strong is modern culture. Japan is reasonably good, although it has lost a lot of its historical heritage (both material and cultural).
Family and friends are where they are....
I want to add four vital criteria : food, safety, housing and politics/laws.
For the food, the best countries in the developed world would be Japan, Belgium and France, followed by Italy. The US and Spain are at the bottom of my list. The Netherlands and Scandinavia tend to have too plain food (btw, I have never seen a Dutch restaurant, anywhere in the world, even in Belgium !).
As for safety/security, the US has a bad image because of guns, and indeed more violent crimes than other developed countries. It's of course worse in big cities, like everywhere else. Big French cities also tend to become insecure. In contrast, I have always felt safe in the UK (apart maybe in Manchester). Belgium is fairly safe, except some districts of Brussels and Antwerp. Japan is very safe, but earthquakes, typhoons, asbestos-filled stations and policemen harassing (some) foreigners made it feel much less safe than Belgium for me. Australia has slightly more crimes than average, strong UV rays (lots of skin cancers), and lots of dangerous animals (some of the world's most poisonous snakes, spiders and jellyfish, + crocodiles, sharks...).
The quality of healthcare is also to take into account. France and Belgium have some of the highest standards in this regard. The UK has completely free healthcare, but less good treatments. Japan is so so (depends a lot on where you go). The US can be excellent if you have money, but unenviable if you haven't. Let's also talk about social securiy. It's abusively generous in Belgium, France, Germany, or even Italy and Spain, but lacking in the US. I think the good balance is to be in Northern Europe or even Japan.
The quality of housing is similar in most of Europe, slightly higher than in the US, Canada and Australia (e.g. there are many wooden houses in the US, easily destroyed during hurricanes or floods, which is unheard of in Europe). Japan is at the very bottom (very big gap). Inside Europe, France and Italy have some of the most beautiful "residential" architecture (i.e. public buildings, palaces and museums excepted). The Benelux has some of the best houses in Europe the countryside. The UK has huge variations in esthetics by regions and neighbourhood, so it really depends where one lives (better in Wiltshire or in the Cotswolds than in Birmingham).
Finally, I wouldn't live in the US because of the legal system which authorises people to have guns or drive a car at 14, but prohibits them to drink until 21, forbids abortion, or allow the police to search your house whenever they fel like it (Patriot Act). Western Europe is very socially liberal and 'modern' on the whole, especially the Benelux and Scandinavia. Japan has a very outdated legislation on many issues.
Overall, I'd say that the UK, Belgium and France are the most liveable, immediately followed by Italy, then Japan. More specifically, the South of England, North of France (e.g. Lille) and Benelux are good. I wouldn't live in Paris because of the people's lack of courtesy and the insecurity. The south of France, and most of Italy are good as well.
The US is very big and may also have good places to live (although the laws I referred above, social security and food don't vary much nationwide). I can't really say as I don't know the country enough, but the West coast and North-east seem to be the best areas. Canada and Australia are better alternative, and typically have cities ranked among the most liveable on earth. The drawbacks of Australia are the lack of history, the isolation, and the strong "island mentality" (read "ignorance of the world") also found in Japan and the US.
Here is a quick rating of a few countries according to my experience or knowledge. Note that "legal system" includes politics, "economy" includes "unemployment", and "people & education" includes manners and general knowledge.
(points from 1 to 5 : poor => ok => good => very good => excellent)
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UK
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History & traditional culture : excellent
Modern culture : very good
Economy : very good
Food : ok
Weather : ok
Security/safety : very good
Housing : very good
Legal system : excellent
Healthcare & social security : very good
People & education : very good
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OVERALL : 38/50 (very good)
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France
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History & traditional culture : excellent
Modern culture : very good
Economy : good (because of unemployment)
Food : very good
Weather : good
Security/safety : good
Housing : very good
Legal system : very good
Healthcare & social security : excellent
People & education : good
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OVERALL : 38/50 (very good)
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Belgium & Luxembourg
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History & traditional culture : excellent
Modern culture : good
Economy : very good
Food : excellent
Weather : ok
Security/safety : very good
Housing : very good
Legal system : very good
Healthcare & social security : excellent
People & education : very good
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OVERALL : 40/50 (very good)
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Netherlands
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History & traditional culture : very good
Modern culture : good
Economy : very good
Food : ok
Weather : ok
Security/safety : very good
Housing : very good
Legal system : excellent
Healthcare & social security : excellent
People & education : very good
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OVERALL : 37/50 (very good)
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Germany
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History & traditional culture : very good
Modern culture : good
Economy : good
Food : good
Weather : ok
Security/safety : very good
Housing : good (a lot of cheap concrete reconstruction after WWII)
Legal system : very good
Healthcare & social security : excellent
People & education : very good
----------------------------
OVERALL : 35/50 (quite good)
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Italy
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History & traditional culture : excellent
Modern culture : very good
Economy : good
Food : very good
Weather : very good
Security/safety : very good
Housing : very good
Legal system : very good
Healthcare & social security : very good
People & education : (very) good (better in the North)
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OVERALL : 39.5/50 (very good)
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Spain
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History & traditional culture : very good
Modern culture : good
Economy : good
Food : ok
Weather : excellent (esp. mediterranean part)
Security/safety : good
Housing : good
Legal system : very good
Healthcare & social security : very good
People & education : good
----------------------------
OVERALL : 34/50 (quite good)
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Canada
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History & traditional culture : ok
Modern culture : good
Economy : very good
Food : good
Weather : good (cold winters, but drier and sunnier than Northern Europe)
Security/safety : excellent
Housing : (very) good
Legal system : very good
Healthcare & social security : very good
People & education : (very) good
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OVERALL : 35/50 (quite good)
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USA
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History & traditional culture : good (NY=excellent)
Modern culture : excellent
Economy : very good
Food : poor (NY=ok)
Weather : (depends of location => good)
Security/safety : ok
Housing : (very) good (NY=very good)
Legal system : ok
Healthcare & social security : ok
People & education : ok (big gaps exist, though)
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OVERALL : 28.5/50 (good)
New York : 33/50 (quite good)
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Japan
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History & traditional culture : good
Modern culture : very good
Economy : (very) good
Food : excellent
Weather : good (sunny, but air too humid + typhoons, etc.)
Security/safety : very good (lose due to earthquake, etc.)
Housing : poor (worst in developed countries)
Legal system : ok (
see article)
Healthcare & social security : good
People & education : good (polite and literate, but poor general knowledge)
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OVERALL : 31.5/50 (good)
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Australia
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History & traditional culture : poor
Modern culture : very good
Economy : very good
Food : good
Weather : very good (too strong UV though)
Security/safety : good (nature at its most dangerous)
Housing : (very) good
Legal system : very good
Healthcare & social security : good
People & education : ok (friendly, but not so well-mannered and not cultivated)
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OVERALL : 31.5/50 (good)
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