For me, being a Manchester United fan, it's tomorrow: Liverpool. If you look at the history, it goes deeper than football, but United and Liverpool are the two teams that represent our cities.
I grew up on this game when Liverpool were more successful than us (in the 80s), but we were still more famous. It's been as nasty as you can ever imagine at times - my mate's been slashed across his face on the way back to Liverpool Lime Street station and I know of much much worse - but we really do have a degree of respect for what each other has achieved. That said, every game I've ever been to has been pure venom with people getting hurt. Sometimes very seriously.
I "look forward to" it in that it's the first I pick out from the fixture list, but that isn't really true. It means so much to win or to lose that I'm a bag of nerves and I'm actually relieved to get it over with.
That's my game.