I have run & played Omega World. To me, it's the best. It ranks as one of the best d20 games ever. One of the best games ever. I bought 2 copies in addition to my subscription copy. I gave one to a friend. He bought another copy and ran the game. We just finished up that long-standing game earlier this year (after an ntermittent 10-year run using 3 different rules sets). I ran it using the campaign presented in Alternity Gamma World. Some of my best gaming ever. I would play or run Omega World right now.
I also have 2 copies of V for Victory. I've never run it or played it, but it has some really great ideas--like the Hollywood option for hit points that is basically now going to be in Star Wars Saga Edition. Ultimately, the d20 engine is probably a little too unwieldy for such a game. Savage Worlds would probably work better for me.
Iron Lords of Jupiter is another great concept. I just got a related product, Mars. I may have to get Iron Lords of Jupiter back out for another look.
It may time for my D&D game to do a little genre-hopping...