They're all very strong. You can't really go wrong with any of those guys. My vote goes to Malzahar though, because in my opinion he's the most fun. He's probably got the highest consistent damage in the game.
I like Malz the most, but that's just because I'm bad at the others. They're all pretty cool. But Malz can clear waves for free with two DRings, and if your lane opponent is too tanky to take out by yourself (such as Galio), you can roam to another lane and break someone there.