My favorite games of the last couple of years are left4dead and Starcraft 2... nothing really comes close for me. Battlefield BC2 / Vietnam is kind of cool but I haven't played a lot.
The one game that I already know will join the list is Portal 2. There is no way it's not going to be awesome.

The only reason I can think of why left4dead isn't as popular as it should be imo is that you absolutely need a team of friends on skype to play with... otherwise it can be really frustrating.