When ranking various venues, I think it is important to distinguish between Game Day atmosphere and Road Trip. Minneapolis, for example, isn't a great Game Day atmosphere (although it is improving), but it is a great Road Trip, at least for me. Factors I consider in Road Trips are:
1) Game Day Experience, including the fanbase and tailgating
2) Ease of getting there/back
3) Other things to do in town
4) Presence/absence of relatives/old friends
Therefore, for me, Minneapolis, Iowa City, and Northwestern are good Road Trips even though only Iowa City has a good Game Day experience. One can do the trips without 8 hours in the car, see friends, eat at any number of good establishments, and see college football live. What could be better?
If I go only with Game Day/fans, I'd put OSU first; despite what others have said, I have not had a bad experience in Columbus and I've been there three times. PSU would probably be second, then Iowa City, then Nebraska. Michigan disappointed me. I haven't been to any of the Indiana schools or MSU. The MSU fans I met at the B1G Championship game were fun to hang out with.