At times I have fun and other time I don't. Sometimes I get the strongest urge to play some champions and it's so satisfying to play them (and hopefully do well). Playing the other games modes is fun as well. Then when I play with friends it gets really fun where I get to play a bunch of different things, we play odd troll comps which sometimes work, we do stupid things and it's fun. I only find it not fun when I'm try-harding, playing my role and when I play the same champion over and over (when I'm try-harding) but that's my fault for not being proficient on other champions.
P.S Dominion is pretty underrated in terms of fun in my opinion, 15 minute game of everyone dying. Not to mention, anything works and no one gets really frustrated over it.