Gotta log another vote for Resident Evil 4.
I think I probably played through every one of the RE games more than once, but I must have re-run through RE4 at least 5 or 6 times. Once you knock it down once and get the Chicago Typewriter, it's just so much damn FUN to plow through again, mowing down everything in your path with extreme prejudice without breaking a sweat. It feels like retribution, especially on tougher characters like Dr. Salvador who gave me such headaches the first time through with wimpier weaponry.
Well, I guess Salvador didn't so much give me a
headache as he did remove my ability to have a headache altogether, but you know what I mean.
The only other game I've played through that much was Dead Space, and for much the same reasons. Chewing through the Ishimura with the best weapons and armor right from square one is immensely satisfying. I think I started over again immediately as soon as the credits were done rolling!