1) Sydney Bristow (Alias)
2) River Tam (Had Cameron from TSCC at first for durability, but I assume Tam's mental abilities carry over to space terrorists or what have you)
3) Fox Mulder - He knows how these guys work. Plus he's been abducted by them, and survived.
4) 4th spot I'd probably have to fit to a tech-god, to balance out the team. I'll go with Marshal Flinkman (also from Alias), because he's also comedic relief, and without comedic relief, no bad ass team can win.
Leading candidates for 3rd spot include:
Gordon Freeman
Morgan Freeman
Jack Bauer (they're space terrorists)
Tiger Woods (can't lose)
Michael Jordon (he's dealt with aliens before)
Vash the Stampede
ksharp
^But figured they were all too OP.