1. Thief 1
2. Thief 2
3. Deus Ex
4. Thief 3
5. Sanitarium


The top 4 spots were very easy. Place number 5, for me, has a lot of contention ;)

No real order

Quest for Glory I-V
Gabriel Knight Trilogy
Alan Wake
Raptor: Call of the Shadows
Arcanum

I just have to go with 5? Man this is going to be tough but

1. Quest for Glory 1-5
2. Ultima 7 Complete
3. The Pandora Directive
4. Baldur's gate 2
5. Anchoronox

in no pacticular order
honerable mentions go to: The Witcher, Thief Gold, Thief 2: The Metal Age, LOK: Soul Reaver, Wing Commnader 4, Ultima 4+5+6, Outcast, Tomb Raider 1-3, Syndicate, and Theme Hospital, actually just about everything on my GOG shelf

In no particular order,
- Thief Gold
- Thief 2
- Deus Ex
- Duke Nukem 3D
- Two Worlds

Like huckleberry, the top 3 were a shoe-in. I know Two Worlds isn't that popular, but there's something about it I just like, I don't know why.

1. Fallout 2
2. Heroes of Might and Magic III
3. Arcanum
4. Commandos Ammo Pack
5. Legend of Grimrock

I should really get around to playing that. I got the complete version boxed for really cheap at the Salvation Army a while ago and then completely forgot about it.

I'd definitely give it a go, since you've already bought it, you might as well :) I really like it, but it is very generic fantasy RPG, some people just can't stand that kind of thing.

That's a tough choice. There are quite a few more GOG games I adore (~20-30), and that's not even counting new favorites I haven't gotten around to playing yet. That said, these may not be my definite top 5, but merely a good approximation. Actually, I'm going to cheat a bit here and list my top games in my top (sort of similar) genres.

Descent 1+2/WC: Privateer - Loved them when I first played them, and they're still a blast to play. The latter is sure is showing its age, but it's still my favorite space sim.

Master of Orion 1+2/Alpha Centauri - They definitely tie for my favorite 4X games, bar none.

Original War/Stronghold - My enthusiasm for everything RTS may have left me long ago, but these are damn good enough for me to still love them despite that fact.

Crusader: NR x2/Blake Stone x2 - Dated, awkward controls and all that they may have, but their wonderful mindless action can still be a great joy nonetheless.

RollerCoaster Tycoon 1-3 - I've been a big fan of Theme Park back in the day, and these are just as good if not better - though I did like the full simulation aspect of TP with stocking, contract negotiation, park sabotage. I know some don't think 3 as that great, but it's different and RCT enough for me to love it just the same. As trivial as it may seem to some, I find that being able to fully ride each and every attraction is very enjoyable.

Ultima Underworld
Ultima Underworld 2
Privateer
Deus Ex
Ultima IV

I made it about 10 or 12 hours in before I realized that for the most part, I was starting to dominate nearly everything and the world didn't seem as big as it was before.

It is definitely good at filling a niche, it's just one of those games that I felt just kinda flatlined after a while.

1. Dungeon Keeper
2. Sacrifice
3. Giants: Citizen Kabuto
4. Heroes of Might & Magic II
5. Sanitarium

1. Planescape: Torment
2. Thief 2
3. Fallout 2
4. Baldur's Gate 2
5. Prince of Persia: The Sands of Time

1. Heroes of Might and Magic 3
2. Sacrifice
3.Sacred: Gold
4.Postal 2
5. Fallout

1. Baldur's Gate 1
2. Baldur's Gate II
3. Icewind Dale 1
4. Heroes of Might and Magic IV
5. Heroes of Might and Magic V (disc-owned, but it's also on Gog, thus makes the list).

There's a which-to-play-if-I-have-time of owned-but-not-yet-played games also:
1. Planescape Torment
2. Arcanum
3. Fallout 1 or 2 (not sure what to dive into if I like Fallout 3 a lot and want to dive into original 2D greatness)
4. Icewind Dale II
5. The Witcher

Edit: I admit to having a bit single-minded games-i-like list (though there would be more variation if classics like Battlefield 1942, Battlefield Vietnam and Company of Heroes would make it to GOG).