Monkey Island of course. And Day of the Tentacle. Oh.. and Kings Quest, Space Quest, Quest for Glory... those were my games..
Dune 2... that was a really good one. Master of Magic too. Anything in the last 2 decades though... nothing really comes to mind at the moment. Don't have too much time for games anymore unfortunately.
Mega Man classic series (2 and 9 are my favorites).
Rival Schools 2 (Project Justice)
King of Fighters 98/2002/XIII
Garou: Mark of the Wolves
Quake 3 Arena
Left 4 Dead 1
Just about all of the Street Fighter series (been playing a lot of IV/V lately)
Crysis - all incarnations of it. It's something I can go back to time and again and still find something new to discover.
Other than that my first game ever played on PC was Doom (the original) then Heretic and Hexen, very similar to Doom.
I'll give any FPS a go and I've played some really good ones - some with excellent game play and naff graphics, some with excellent graphics and naff game play.
I also had a period of playing Command and Conquer and Civilisation and whilst totally immersive and captivating, they both had a tendency to destroy my social life (not to mention robbing me of sleep lol).