Now I've never played Half-Life 2, but I have played Marvel Alliance 2. It's only reason then that I vote for....Half-Life 2.
Reason: I really didn't like Marvel Alliance games. Yeah, you can play as loads of different Characters, you can shoot webs as Spidey or claw people as Wolverine, but I have played Half-Life. Now if going on that is anything to go by for the second one, then it's only fair I vote for Half-Life 2. I LOVED the first game and I've only heard better about the 2nd, but never got round to playing it. I can only guarantee that I will now though. But yeah, even having played M.A, I still give my vote here to Half-Life.