My top five and the reasons why, I would put ten but I can't think of any other ones at the moment for some odd reason.
1. F.E.A.R: Visually stunning, atmosphere really scared me along the way... the horror elements work so well with this game and the shooting sequences are just totally out of this world entertaining.
2. Psychonauts: May be a weird choice for number 2 but not for me, it's based on my favourite genre - platforming... but wait theres more, this game is so funny - it had me laughing non-stop and it has some of the best gameplay i've ever played, i'd even go as far as to say its better than Mario 64 - but thats not for PC so I can't list it.
3. Half Life 2: Very graphically impressive, awesome gameplay - very good story. It would've been my number #1 if it weren't for the horrifically bad ending sequence... seriously. Apart from that the puzzle solving element and shooting, even the vehical segments were a breath of fresh air.
4. Need for Speed - Most Wanted: Best driving game ever, not only does it look fantastic but it has some of the most fun driving as well - the chase element from the cops is even better than Grand Theft Auto's. The races are great as well and the sheer size of the area you can drive is just so impressive.
5. Grand Theft Auto San Andreas - This one took up a lot of my time, the fun factor mainly, mindless killing and driving around. Missions are good for killing time and to intentionally let the cops chase you is always a laugh. The only thing I did not enjoy about this game is the fact that there are no children on the streets, Why the hell not? why can't I murder some 5-10 year olds as well.