I want to buy some new characters with my RP, can I get some recommendations for good legends? Maybe one for every role? Thanks alot!

Hecarim isn't played too often these days and makes a solid solo top.
Ahri dominated mid lane all day, but if you want a fun neverplayed, try Viktor.
Jungles currently belong to champs like malphite, Jax, and Lee-Sin
Ez is supposedly best ranged carry, but graves and varus work as well
Soraka used to be queen support, but Janna and Sona do well also.

Jungle, does not quite belong to them
belongs to mao, shyv, and nunu
Mid try and have some fun with morganna
Ez not the best ad ranged carry, and nobody ever uses varus adc. The best ranged right now, IMO, is corki and in a close second graves. Also looks like Sivir might come back into play.
Support thats most Op right now would have to be Taric and Sona poke.
Solo top I would try and go for vlad or irelia. Jax isnt a bad pick either. People like Rumble too but.. ive never played him.

The OP champions will be different for each map (Summoner's Rift, Twisted Treelines, Crystal Scar).

Twisted Treelines tier list:
http://na.leagueoflegends.com/board/....php?t=1568877

Crystal Scar tier list:
http://na.leagueoflegends.com/board/....php?t=2152948

I don't know if someone made a tier list for Summoner's Rift too, I'm asking it:
http://na.leagueoflegends.com/board/...2#post28933902

I Personally think you should use Ahri, Being an assassin mage, she is a mage with high burst and dps output. she can also do great ganking for pvp. her weakness is her amount of mana to start out with, and the targets with more health like Darius, or Graves, and Blitzcrank, because she specializes in taking out squishy targets, and taking people by suprise. If you DO decide to use her, i recommend learning how to use her first, because she is hard to play if you dont know how. i looked at the tips and items, watched the champion spotlight, and even read her wiki just so i could learn how to play her to her best. oh and by the way, you also have to know how to land skill shots, because two of her moves are skill shots, and one is vital for setting up her combos

How do you define 'Best and most OP'? If we're going off a measure of win percentage the tops (as compiled by lolking anyway) are:

Top: Rumble
Jungle: Amumu
Mid: Zyra
Support: Taric
ADC: Ezreal.

I seem to find that Garen is stupidly OP. Personally I favour Amumu; he was fairly OP pre-nerf and quite strong post-nerf.

As Jungler, get Amumu (possibly Nunu)
As mid, get Zyra, Diana, Cass, Kennen, Galio
As top, get Kennen, Wu Kong, Garen
As support, get Sona, Galio (possibly Nunu)
As ADC, get Ezreal, Miss Fortune, Graves

Hope to god your team has AOE, and more importantly, AOE CC. 4 AOE Champs can win 4v5s., especially if it's something stupidly powerful like Galio Amumu Kennen.

Diana is especially potent in this regard, because she draws everyone in. Forcing the entire enemy team to endure a full Garen Spin or Kennen Ult or whatever AOE you have undoubtedly causes a lot of damage