He carries around a fricken briefcase, how cool is that?
As an adventurer it makes no practical sense (even for an adventurer seeking to be a doctor), but he totally rocks it.
Also he's just a generally fun character with goals so impressively different from the usual battle hungry suspects.
Second would definitely be Hisoka though. He's good at what he does and he's off in just the right kind of way.
Leorio is my favourite aswell, I love how much he cares about his friends.