The one that you can play the best.
But if I had to name one, I'd probably say support. It might not be the case any more now with the new champion select, but at least back in previous seasons no one wanted to play support, so there was a very good chance that whoever was left with it didn't really want to play it and wasn't very good at the role.
That meant that if you chose to play support and were good at it, you were making a much bigger difference in the overall skill level of your team than another role.
(Example, you are 10% better than the other player who wants to mid, but 70% better at supporting than the guy who doesn't want to play support at all.)