Back in the day, the Leader had a great rivalry going with the Hulk...that whole "brains vs brawn" thing.
The only thing missing is that it never really got personal. It always seemed the Hulk just got in the way of whatever diabolical scheme the Leader was cooking up.
The rivalry between the Hulk and the Abomination,while not as old as the Leader's, always seemed more personal as their conflicts were always physical. And of course, it became VERY personal when we found out that Blonsky had murdered Betty.
When the villain wants to take over the world, that's one thing.....when he goes after the hero's girl, that's a whole new ball game!
So Abomination gets my vote.