It's actually hard to make a tier list for you, especially since you're level 20, but what you see at higher levels of play and in competitive is pretty much this:
BROKEN Tier:
-Gnar, you can pretty much pick him into any matchup and he will at worst be able to farm with Q from insane range. He's also an extremely oppressive laner if the matchup allows him and can turn the laning phase of most toplaners into a living hell. He also does a CRAPLOAD of damage without building pretty much any damage items. You're going to see him banned or picked in every single competitive game these days, he's just too strong right now. He's pretty fun to play but not really easy (not really hard either), you might want to check him out.
Awesome Tier:
-Irelia, a strong mobile bruiser with a pretty good laning phase (can also handle Gnar and most ranged matchups if played decently) which is great at diving carries in teamfights as you can pretty much build a Trinity Force and then go tanky and still be really useful.
-Rumble, not the best laner (he is GOOD in lane, just not the BEST), does a shitload of damage but is kinda hard to use.
-Lissandra, strong poke, really safe laning phase, WAY too much croud control and untargetability.
-Maokai, REALLY easy to use, a lot of CC, great sustain and good damage early on.
-Jayce, you mostly see him mid these days but he's still a great ranged toplane lane bully with great damage output.
-Kassadin, great damage and mobility, is innately tanky against magic damage.
Pretty Good Tier:
-Jarvan IV, good burst, much utility, scales well.
-Dr. Mundo, really safe laning phase, great sustain, good damage. Becomes one of the thickest tanks in the game.
-Sion, can farm from range and is pretty safe in lane, has a lot of crowd control and becomes extremely tanky.
-Nidalee, ranged, has sustain and good burst.
There are a few more you might want to check out, Riven for example, but that's pretty much it.
Remember than at level 20 you pretty much have to use what you're good at, what works against the people you're used to play against. If you can stomp people with Garen and Darius then do it, that's completely fine.