My favorite was Decent 1 and 3.
Followed by Counter-Strike, which brought round based action into my little world. CSS cought me for a while, but once I found Hidden:Source I have never touched CSS again.
I'm actually going to revise my previous best to Mass Effect 2. I bought it during the steam sale and I just couldn't stop playing it. The story was as gripping as any good novel and the gameplay fantastic. 10/10 for me.