For me, it was No One Lives Forever & Hitman: Codename 47. I bought the 2 games on the same day (I was feeling gamy that day). My first thought was that No One Lives Forever was gonna be just another FPS, and Hitman: Codename 47 was gonna be awesome. Let me tell you, was I wrong or what, No One Lives Forever turned out to be THE BEST FPS EVER and Hitman was one of the worst game I have ever played!
After a few months, I got rid of Hitman (garbage bag was quite happy receiving this trashy game)
and once every year, I still play No One Lives Forever!
NOLF! Quality game. Mate got that bundled with a graphics card. He recommended it to me and leant me the disk, and I almost didn't install it. Every game he's recommended to me has been awfull. Was just very very bored and popped it in the tray. Superb game. Great story, funny and a solid shooter. Austin Powers style spy thing with a female lead and explosive lipsticks. Dunno if you played it online, but had a really good community. NOLF2 - that was a real let down.
Gal Civ was another what the hell moment. Disciples2 was another. Another one that did it for me was StarTopia - not normally a Sims fan, but that was actually a really nice suprise. Its a sim with humour - its dated - but still can raise a smile if I replay it.