Purely for speculative purposes, I decided to try using Facebooks new "question"-thingie to figure out what old game people have as their favourite, not necessarily think the best, just like the most.
To make the data more interesting I thought I might as well see what games the gogers favourites as well:
http://www.facebook.com/home.php?sk=question&id=198925720146856

Mine is most definitely Homeworld, and have stood by that game for at least 6-7 years now.

Nearly all the games I own & love are pre-2004, so this hardly narrows it down.

I'll probably go with Sid Meier's Alpha Centauri. STill in my mind one of the best games of all time, all genres.

Runner-up: Arcanum, though it has stiff competition from many of the other cRPGs offered on this very site (and a couple that aren't, but are prodigiously requested).

Tie Fighter for me no doubt. Ideally a remake ala Monkey Island Special Editions, but wouldn't say no for a GOG version either.

A shame we are limited to PC games. I'll take Might & Magic World of Xeen and Pharaoh then, of which one still isn't on GOG...

Feel free to add new answers to the question if your favourite game is missing:
http://www.facebook.com/home.php?sk=question&id=198925720146856

Mine's definitely shattered galaxy, buggy as hell MMORTS, but still loved the shit outta it.

Warcraft 3 probably.

I can never decide what my favourite game is (although it would definitely be pre-2004).

I can't even decide on favourites for individual genres!

My favourite would be that game which hasn't been released on GOG yet.

You don't mean...

GAME X???

Deus Ex and Morrowind are tied for my favorite game of all time. Coincidentally they are both PC games from before 2004.

Just one? That's a very tall order considering most of my favorite games are all pre-2004. :P

I'd say System Shock 2 and Morrowind are tied for the sheer amount of joy they cause me.

"To be on top" or "Leisure Suit Larry" (Part 1)... that were great! Still loving them