I always hear people mention something/someone "not being fun to play against" but I quite legitimately never see anyone say that something is "fun to play against"
And that's not even an exaggeration
Is this just a blame philosophy created by players so they can feel better about their loss? I don't understand
any champion that sucks when they're behind being played by somebody who doesn't know how to play them and will inevitably feed
jax vs irelia in season 3 was really fun
both of you hit most of your powerspikes at the same time, both of you had added kill pressure with ignite and 1 kill could decide the entire lane, but neither side was too heavily favored, jax had a slight advantage but it wasn't so massive that irelia couldnt win
Any champion who you can appreciate when you get outplayed.
Orianna and Nami are probably two of the best examples. If you get outplayed by either it genuinely feels like it's a fair outplay rather than being cheated by a cheap mechanic. For instance, if you get 'outplayed' by Annie by her flash combo or by Fizz doing...anything, you feel cheated rather than 'outplayed'.
Essentially what makes a champion 'not anti-fun' is their outplayability on both ends. Unfortunately as you hint on, it's been abused by players to feel better about a loss. Thus I have to use the term 'not anti-fun' rather than fun.
Some champs are just notably more annoying than others to the point where it feels like they're actively devouring fun. Like Riven.
I'm weird, I know, but the fights are often close and it's such a rush regardless of the outcome
It's fun to shit on Rivens and Yasuos
(edit) haven't gotten a jax/riven matchup since fervor was removed, so it might be different now
Champion with all skillshots, like Xerath, Ziggs or Vel'Koz, champions who can be outplayed if you dodge them.
I have fun when both players are cutting it close or outplaying each other. Kled is a good example.
.... looked at every champ, and uh...... the answer is always karthus.
I actually enjoy playing against 
. Taking the risk to kill his barrels to slow his push and get a little extra gold is always fun to me no matter how I'm doing.
