@ bolded, this , i've been playing on PC since PCs were made (Desktops * )  , so i can't tell which is my favorite.

*yes i know some smartass will say "wow you had a giant sized PC in your room?!"

I disagree. I think its more to do with the way generations work on console and the longevity of PC games. On a console you've got an approximately 5 year range (give or take) in which games are going to come out for it and they are specific to that console. On PC you've got 30+ years of games and they all fall under the same banner of "PC Games". For example, your favourite list of PS3 or 360 games isn't competing with games that are 5+ years old because the consoles didn't exist back then. I think if you asked people what their top 5 favourite console games were, you'd find a large number of them would have games from last gen or even earlier and the lists would generally lack PS3/360 games.

This also leads to games needing to stand the test of time. While Dragon Age might be an excellent game, am I going to come back in 10 years and still think its as good as it was back when it was released? Many of the games mentioned so far are still played today and even have active modding communities. While a game released in the last few months might be really good, I cannot decide if its one of my "Top" games without that factor.

Now, to answer the original question, I'm going to have to be somewhat generic:

Baldur's Gate Series
Max Payne Series
Sierra and Lucasarts Adventures from the late 80s and 90s (I seriously can't break this apart as there were so many good games)
Deus Ex
Mafia
Eye of the Beholder 2

And I shall stop there for now. In my 20+ years of PC gaming, I've played so many good games that I could spend all day making a list