I don't have an order

Diablo 2
Warcraft 3 (DOTA!)
Starcraft
Oblivion
L4D2, totally crazy about this game lately.

1. Fallout 3
2. The Orange Box
3. Deus Ex
4. Half-Life
5. Diablo II

Then we have BGII, ME2, Planescape Torment, Fallout 2, Starcraft, Warcraft III, ect.

It would probably change every time you asked me but,

Starcraft
World of Warcraft
Half-Life 2
Battlefield 1942
Civilization IV

1) Crysis Warhead
2) Fallout 3
3) Phantasy Star Online: Blue Burst
4) Crysis
5) World of Goo

Ohhh world of goo is such a fun game too!

Also, is it just me. Or do older games mainly dominate this list? Is it just me. Or do they not make PC games as good as they used to?

In no order:

Warcraft 3
World of Warcarft
The Sims
Roller Coaster Tycoon
Left 4 Dead

Err... this is hard. 5 top games that I mostly would name, can't get them into any order though (and I'm counting a few franchise rather than individual titles):

Half Life franchise
System Shock franchise
Deus Ex (pity I can't say franchise)
Grim Fandango
Thief franchise

basically I prefer a first person perspective for immersion but I want a compelling atmosphere and narrative with some light RPG elements - with the exception of Grim Fandango, which I'll take in any perspective I can.

A third deus ex is coming this year I think. Could have sworn I saw something in a gameinformer.

There's a third on the way indeed, and I dearly hope they don't mess up this time.  With 2 they both tried to push a modified Unreal engine too far while at the same time cut back on level size, etc. to fit it on the Xbox and the result was just terrible.  Horrible, tiny levels, useless stealth despite and actually quite advanced lighting system, broken character rag dolls and no sense of scale, ever (well, apart from the openning cinematic, which was ace).

The only gripe I think I have with MS entering the console market is the indirect impact they had on Deus Ex 2 and Thief 3 which negatively hurt both titles, particularly Deus Ex 2 vs what they could have been if simply made properly for PC (which is what they should have been at the time).

Now though, with PS3/360 have some decent horsepower hopefully they can get back to what made Deus Ex great.  I'm not very confident though because I suspect they'll make it more of a shooter than anything else.