Go to Genki Burger near Waseda. They use full half pound patties, plus you can get it with just about anything...I liked mine with gouda cheese and bacon. The chef is kind of a jackass, but the food is good. I wonder if it's still there...Hit the Waseda subway station, exit on Waseda dori, head towards Takadanobaba and it'll be on your left side next to a bank at a 3 way intersection. The canopy says Genki Genki Genki (in kanji of course) and is next to a konbini...I think a Family Mart. I went there last June, so I'm hoping it's still there. G'luck.
Only bad thing...no fries. The omelette rice is good though