Deus Ex would make a really good movie. I agree, the SFX would be a constraint on the costs of making it a decent series, though it could be interesting with a lot of spiralling plots and twists.
Fallout would be an awesome series, a bit like the Walking Dead comic maybe, with the surviving factions eking out a life and possibly competing or teaming up with each other, with bands of raiders thrown in. I'd definitely watch it. Could take a few cues from Mad Max I think, like the game already does (and vice versa probably).
I'd love to see an XCOM movie, it'd be something like a military drama crossed with X-Files or something I dunno. The awesome thing about XCOM games pacing is that you're never really totally on top of things, each small victory feels even smaller when the next huge thing happens or arrives. And that endless desperation is what keeps the game tension up. I can't remember that cutscene in the game where they're all celebrating and the chief scientist chick is just shaking her head and walks off...cliche but awesome.