I'll start with Fire Warrior. Yeah, that old 40K fps that no one likes. Playing the first levels, I thought it was a boring game, but later on I changed my mind. This game fucked with my very soul, especially when the Chaos Marines showed up.
Those Chaos Raptors, I can still hear their mindless screams as they charge at me trying to claw my face open.
Metal Fatigue. The gameplay was pretty crappy, but it had so many awesome ideas. Multi-layered RTS maps, Robot construction, Stealing tech from other players, etc.
Also damn the intro movie was cool.
Arx Fatalis. Pretty much Ultima Underworld spruced up with an interesting story, setting, characters and a good sense of humor. Also, the spell system is amazing.