Grand Theft Auto IV (w/ DLC) if I'm brutally honest. I did like the earlier games but now IV is out, the older cities feel too small, III and VC are crippled by bugs and issues and San Andreas just doesn't entice me like it used to.
Saints Row 2 would be up there, but PC support was seriously lacking and it felt too short, not to mention that the gameplay wasn't the greatest. I'd put it on par with the GTA III era games in terms of my like of them.
Driver? Nah, I think that the brilliance of Driv3r was never capitalised on (Not played Parellel Lines, so take that with a grain of salt) and as such, for me, it's out of the running. The next game in the series (San Francisco) goes away from what could have been a great direction for the series, one that could have brought it back into the running. I have fond memories of Driv3r, but I think it was more rose tinted specs than anything.