I guess that varies depending upon your definition of "best." Most willing to follow their team to the ends of the earth? Most accomodating to other fans? Most knowledgable?
Most affluent, most likely to sell out a bowl game, Notre Dame has got to be number one. Domers are spread pretty much throughout the entire US pretty heavily. Most knowledgable would probably be OU fans. I have had several discussions with OU people and been really impressed with their knowledge of not only their program, but our program as well. LSU fans are pretty much the biggest partiers and the coolest to tailgate with. I am going to see the ASU-UO game in October, and I have been given a warm welcome by all ASU fans and have been invited to drop by about 5 or 6 different tailgates. Super people.