I recently bought Malzahar but only after trying a LOT of other champions. And I've found that some champs are just better for 5v5 over 3v3, like Ashe, I'd pick her in 5v5 but in 3v3 I make so much less money it makes little sense.

Malzahar works out nicely in both, but obviously makes more cash in 5v5, though I can pick people off quicker in 3v3.

I was just wondering if people have a recommended champ dependent on what sort of match they're playing?

Pantheon & Yi very good on 3 v 3.
Amumu very good on 5 v 5

Just look at Elemintz tier lists.