Well an actually anti-fun to play champion, is someone who's straight up frustrating to deal with.
Imho a fun to play against champ can't really exist per se, every champion is frustrating in his own way, but there are those that are bearable (like for example an Ahri, you know that if Ahri beats you, you probably did some kind of mistake). Usually champions that can't just flash (or dash maybe) + combo you to win aren't frustrating for me, and it's usually fun to play against high skillshot oriented champs since you can dodge everything (like ezreal). If I had to make a list I wouldn't really be able to make one, but I can say that champs like Riven, Yasuo, Lee, Vayne, Tahm, Annie, LB are frustrating for me since I don't feel like I've been outplayed most of the times both because they are incredibly safe (and all of the ones I listed are) and because they can dish some nonsensical damage, if I play against an Ezreal though, I know that if he lands a skillshot it's because he was better than me in 99% of the cases, same with most really sillshot oriented champs, if I get snared by Morgana or grabbed by Thresh I know I made a huge mistake since I've played them and so won't feel as frustrated as I would if a Malphite + Yasuo ult hits me, since I can't prevent that even with good positioning, no one can prevent a Malphite + Yasuo wombo withouth incredibly good mechanical skills, game knowledge and reaction time (and even then, if you lack some kind of mobility or flash is on cd it can't be done).
I know my answer is pretty vague, but I can't really think of anything more defined, since every champion can be frustrating and anti-fun to play against given the right circumstances.
TL;DR Skillshot oriented champs are more fun to play against than point-and-click oriented ones, you actually feel they deserved the win if they landed a hard skillshot since you know that skillshot is hard to land, on the other hand if a WW ults you and you don't build QSS or don't have a spellshield, you feel cheated by the game, since you can't do anything about it.