Worst fans I've ever experienced personally were Michigan fans. They used to regularly walk around campus like they owned the place, even in years they were bad or mediocre. It was a regular occurrence to see them starting heated conversations or some fist fights. Fortunately things have calmed down in the last 10-15 years as Michigan has delved further into mediocrity, I haven't seen the bad behavior as much or at all...but we also haven't played as much.
Some of the best fans were Utah, Nebraska, and any military academies. They were polite and respectful in the times we've played them. Georgia fans were fairly good too, but absolutely awful about garbage. Campus was thoroughly destroyed when the Bulldogs came in a few years ago...beer bottles everywhere, smashed glass on the sidewalks and roads, piles of cans... it's like they've never heard of a garbage can.
Worst fans I've heard of but didn't get to experience has been Miami fans. Family went down to Florida for a game back in the day, I guess it was bad enough they said they'd never go back. Fans were very confrontational and rude. There were Cane fans dumping beer on Irish fans, and I guess some even got pissed on. I heard some bad stuff went down the last time we played them in 2017 too, abusing fans and something about Devin Butler's handicapped family member getting harassed or something. Pretty low class stuff either way.
[EDIT] I was wrong, the Devin Butler thing was FSU and not Miami. A fan threw alcohol in the eyes of Devin's handicapped brother and 5 year old daughter. Either way...screw Florida schools I guess.