I mean, every fan base has good and bad so this is more about individual experiences...
Worst game day experiences I've had are at Newcastle and Dogs home games. Newie has such a huge element of actual subhuman filth that rocks up to the game, and we all know about certain elements of the Dogs fanbase.
Both seem pretty good on the whole in terms LU though. Probably because the ones I've had the bad experiences with aren't actually literate and still use Nokia 5110s, and therefore cannot work out how to access the internet.
Raiders fans are pretty bad too. Not so much in terms of matchday behaviour or anything...but they're right up there as whingers, and the way they seem to jump ship en masse if the Raiders are travelling worse than the Brumbies can't be ignored. Mostly good blokes on here though
Panthers fans at the footy seem to never actually know or understand the simplest rules of the game. It's quite funny, but it gets pretty old quickly. They're up there with the Raiders on the whining scale too. Don't interact with very many on here but I do see a bit of that whinging.
And Parra fans are legitimately mental. Check out our forum semi-regularly, I can promise that lunacy runs through most of the fan base. It can be pretty tough being one sometimes tbh...over the course of a match the people around you will want 17 players, the staff, and their extended families outright killed, and then if they score a try five mins later all 17 of those players are literally the finest humans our race has ever produced. And then there'll be a knock on and the nooses are being tied on again...