They came out in different months.

I think Halo 3 wins out a bit in most departments, mostly because of the online co-op, theatre, and forge modes. I'm sure you can do a lot more "forge" stuff with Half Life 2 on the PC, but, well, I don't have the PC version. Portal is pretty awesome though. And any company that neglects to add an option for legacy controls loses several points, in my book.

PS. Can somebody tell me what the hell is wrong with my avatar image? It used to be the Phalanx guy but now it seems to randomize every few days. It's kind of neat, really.