I think for me these days the true magic comes from a game which does something new and does it well. I've played RTSes, I've played FPSes, I've played flight simulators, and if another game comes out that happens to be just another one of those, it's not going to do much for me no matter how pretty it is. But if a game really stands out, I'll keep going back to it.
Highlights for me in recent years:
- Shogun: Total War and Medieval: Total War - Both combined Risk-type strategy with real time tactical battles.
- XIII - It's an FPS, but the whole comic style thing really works for it. Plus the multiplayer mode where you all go hunting the Death absolutely rocks at LAN parties.
- Planescape: Torment - Best RPG ever (I don't care what the poll says). Certainly the best computer RPG I've played yet at actually giving you the chance to role-play and having your decisions really affect the game. I've still gotta go back and try to be chaotic enough to join the Xaositects.
- Republic - I don't know what it is about this game. It's basically a political simulation (how many other games fit into that genre?), and at first glance looks kinda boring, but I still find myself playing it for 4 hours or more at a time.