My vote is Iron Heroes. I feel like I'm somehow less of a role-player for having never played in an extended fantasy campaign. By the time I had sorta gotten the hang of DnD, we moved on to other things. My character concept is totally boss; sexist female warrior from a female-dominant warrior society, like Xena meets the Dad from All In the Family, with all the old prejudices reversed.
Rogue Trader sounds pretty cool. I guess I'm voting for that second, but I know nothing about Warhammer 40,000K.
Zombies and post-apocolypse is cool, but I feel like it has to be hard to have a whole zombie campaign. Personally, I like my zombie setting pretty damn bleak, and I doubt anyone would want to play a long slow slide towards inevitable debate. The second we started negotiating with some sentient zombie, the whole thing would get a little to "Day of the Dead" for my tastes. I prefer the undead to be mindless and hopelessly overwhelming, neither of which I expect is easy to pull off for extended gameplay.