Pick the one you like better; the "best" champion changes constantly, so you'll be better as a player sticking to a champion you actually like and getting good at them, rather than just switching to whatever's "OP".
There's a prevailing opinion that Quinn is OP and Aatrox is terrible, but I've faced some terrible Quinns who don't know how their champion works, and some really good Aatroxs who take over games. The upside of Aatrox is that a lot of enemies don't really understand what he does, and therefore won't expect his damage, etc. Aatrox is probably getting reworked sometime in the future, so I wouldn't bank too much on him, though Quinn also might get nerfs. Both champions have pretty strong splitpushing and can win games that way; Quinn also has really powerful roams, but Aatrox has powerful towerdives due to his revive passive.
If you like Garen, and are looking for another juggernaut, I'd probably wait until the next champion release when (I think) Darius is getting a price reduction. Trundle, Volibear, and Olaf are also really good lane bullies. If you want to win games through splitpushing, Tryndamere is probably the best option. Xin Zhao is broken right now. Fiora makes everyone on Boards cry. Rammus is surprisingly powerful in Top right now. Malphite is rock solid. Basically just pick what you like and get good at them