Hands down it has to be Herc!
I finally have the Devastator I wanted as a kid.
I really enjoyed the entire experience of getting him.
The anticipation of each piece coming out, and when they did, assembling them for the first time.
That 6 months was the best 6 months of collecting for me in a long time.
While I buy the Hasbro stuff, it is often weeks (sometimes months) before I bother opening them, and carving out a spot on the shelf for display.
Not so with Herc.
Each piece was put up immediately in bot mode for a while before the next piece came out. I'd switch the display to some form of combined mode for a few weeks, then back to individuals. It was fun.
I've even enjoyed many of the 4th party add-ons for him.
I bought the Rage set (though I don't have it in at the moment), and I have the purple barrel in my PoL.
I'm even considering some of the Shape-ways add-ons.
The whole thing has just made him an amazing figure with a lot of choices for display.
Herc has the power!