If I only get to pick one, Megatron wins, hand down. (pun intended)
He's one of the most enjoyable TFs ever because of the many, many modes, though he's never felt like Megatron to me.
If you get him in his Galvatron colors, he makes a great Classics Hun-grrr!
I just recently got RID Scourge, and he's pretty cool. I'm on kind of a Nemesis Prime kick at the moment, and really dig him.
I like RID Prime, and as mentioned his combo mode with UM is sweet. Like Megs, he never quite felt like Prime to me, but he's a cool bot none-the-less. I did a custom head and redid him into Alternators Inferno.
I really like the repaint of his titanium version into Hot Spot too.