I just play what I find to be the most consistently fun. If I play a new champ and if it's a fun champ who's playstyle I like then I'll play it again and again and again until I get bored. The small few that I don't get bored with become my mains and I then play them the most often. One thing I've noticed about my mains is that I find them to be funny. They make me laugh whether I'm feeding or carrying (doesn't happen as often as I'd like yet).
So far these champions would be Bard and Teemo. Those two champions never get boring for me and they almost always make me laugh regardless of what's happening in the game, especially Bard. lol
Other people likely pick their mains differently than I do but LoL is a game, it's a competitive one sure but it's still a game either way and games are supposed to be fun. That said, I'd rather main something that I find fun and enjoyable than something that just happens to be the current OP or something. I mean, whether it's OP or not makes little difference because if your champion is fun for you then you'll tilt less and master that champion faster and once you've mastered a champion and can play them well then you can even stomp your counter-picks because most likely your opponent won't be as skilled on that counter pick as you are on your champion making their pick wasted.
TLDR: Main whatever you enjoy playing the most and won't easily get bored with. If your champ can make you smile and have a fun time even if your 2/9/2 then that one should be your main because after enough games on that champ you'll consistently start going 9/2/9.
I hope that that helps. :D