I was in Cincinnatti a few weeks back. Reds fans are top notch. Very friendly...it was vs. Pittsburgh...no fights etc. Great park. Reds are my pick to go all the way.
I think the Cards, Tigers, Reds, Buccos and Mets have the best fans I've been around. Nats fans are nice too. I think KC has a nice group. Never met a bad Royals fan. Just not many.
The worst fans are Phillies fans and there ain't no second place. Red Sox fans are way overrated. They have a ton of kids who love 'em....but their 20 y/o something fans are insufferable. Yankees fans aren't as bad as most people think...they're just stupid. Don't know jack s*** about baseball.