ALL OF THE ONES ON THE POLL.
(And the medium-weight one from Terra Nova: Strike Force Centauri)
Anyway...I doubt I would fit into a 40K Space Marine power armor, no matter how badass it is, and Fallout power-armor is flippin' sweet, but you mentioned the Fallout 3 ones, and even though I love my T-51b, I think I'd rather have the model from Fallout Tactics.
The Crysis one can cloak which is a huge plus, but I've fantasized about having Samus' suit - but again, we'd need to be talkin': Metroid Prime 1? 2? 3? Fusion? Which suit do I get? Does it come with the gunship? It probably doesn't.
The Section 8 power armor looks really awesome, and the jetpack is sweet, but it just feels like a weaker version of the armor from Tribes, and the HUD really pisses me off compared to, say, the one from Terra Nova.
Maybe an Elemental suit from Battletech, but I might as well just go for 40k armor at that rate.
Yeah, ok, I think I gotta go with Samus.