OTP's of pretty much any champion can and have made them work pretty much up to diamond or so, as their raw skill can win them even 'counter' match ups. If your ambitions aren't diamond+, then no matter what you choose, it's going to be viable. Even if your sights are beyond diamond, most champions are still pretty viable (and the ones who aren't currently will in theory eventually be tweaked to be made so). Of the ones you mentioned, garen is the only one that stands out as one known for being poor in high elo (since he just gets kited and can't do much about it), and doesn't have much to offer if he can't get on anyone.
I wouldn't necessarily recommend being a OTP, particularly if you enjoy multiple champions. You can main whoever, but theres no need to go all the way to OTP status and you can still play the others somewhat. If you're having difficulty choosing, it's probably a sign that OTP life isn't for you, and it's really when you play a champion and suddenly feel like you always want to play it and it just comes naturally that OTP life begins, rather than as a conscious decision.