This was an extremely tough one for me, as these are two games that I have played endlessly. In the end I had to go with Halo. As great of a game as RE2 is, I can't rightfully say I enjoyed/played it more than I did Halo. I still play Halo to this day, and so do thousands of other people. The online servers are just as full as the day it was released. RE2, while an amazing game that will always remain one of my favorites, can't rightfully go over Halo for me here.
I voted Halo, begrudgingly. I wouldn't blame anyone if they voted for RE2 though, they're both great games.