I'm thinking you should have named this forum "What genre of MP games are you good at?" since Multiplayer is not a genre but rather a game mode.
Anyway, I'm usually okay at FPS or 3rd person shooters once I get through a few matches, understand the controls etc. , i won't do exceedingly well, but I don't usually get my ass handed to me.
With RTS's I can hold my own but will often get my ass handed to me by my best friends boyfriend, mostly because he plays solely RTS's.
Racing games I'm usually in the top 3 places for any given race, but I find them repetitive after 3 or 4 races.
Don't ask about fighting games, I don't want to talk about it.
So matter of experience here, the games I find I'm best at are platformers and racing games, so in online play, I find myself placed higher than I would in genres I spend less time playing like shooters. Being good at one game online usually goes hand in hand with playing that game all the time.