Man, it's a tough one. I really like all of the suits for different reasons.
VI is definitely my favorite right now. Love the look, and the new weapons are awesome as hell.
V is just conceptually amazing. A portable suit is great, but then again it does seem a bit weaker than others.
III and IV are pretty much the same in abilities, and the looks are fairly similar, too. I do prefer IV's design more though. Much sleeker.
II has that awesome metallic look, plus it becomes War Machine!
I has those great flamethrowers, plus just kicks ass. It's design is awesomely low-tech compared to the others and really the most imposing.
Hell, I love them all! lol