Fortunately (?), I liked so many places enough to not have a favourite. Also, my trip was done in early summer. The landscape/attraction is totally different. One of my stops was Sakata which was the main filming location for Departures, the first Japanese film to ever win an Oscar for Best Foreign Language Film (?). If you have watched the movie, you will agree with me that it is not the best place to be trapped in winter.
Besides Sakata and Matsushima (in Sendai), my loop of Tohoku involved less well-known sites so I'm afraid my information may not be very useful.
Perhaps there is one. At unmanned train "stations", do your very best to find a box that will verify where you have boarded. This ticket will be called a 整理券. Without it, you will have a tough time explaining yourself to the train driver/conductor (often, it is just one person doing both roles) when you alight. You might also end up paying a significant fare.
Of course, this is a non-issue if the train ride is within what is stipulated on the pass you have purchased. For buses, the 整理券 (if required) is to be retrieved after you board. I don't think I have come across any trains that work that way but you might want to do some research on that if you want to be more prepared.