Let's say you could forget everything you know about 1 game so you can truly experience it for the first time again. Which game would you forget?
Also, since I know some people will try to overanalyze this, let's say that you remember the game itself (that it exists) and that you like it, so you have some drive to pursue it again.
I would forget Resident Evil 4. This game's awesomeness blew my face off when I first played it way back on the GameCube, and I still play it here and there on the Xbox 360. The combination of action and horror was near perfect, with phenomenal atmosphere that can still get me even after all this time, and being able to upgrade and buy new weapons added even more fun and reason to replay it. On last count, I've played this game well over 50 full playthroughs, making it my most played game of all time, at least in terms of times completed. Reliving this experience for the first time would be nothing short of amazing.