Planescape: Torment.

With close runnerups being:
Vampire: Bloodlines, Arcanum, Drakensang: River of time, Gothic, Skyrim & Morrowind

And other notable games being:
Divine Divinity, Divinity 2, Drakensang: The dark eye, Fallout 1-2 & New Vegas, Gothic 2 & 3, Risen, KOTOR 2, The Witcher

And not played being:
Fable 3, Arx Fatalis (though I own it), The Witcher 2 & Gothic 4

At the moment - Skyrim, but it is the game I am in the middle of playing now also. It is closely followed by Witcher 2, New Vegas, Arcanum and Star Wars: Knight of the Old republic. (Oblivion also, but it is for now jousted by Skyrim...). Though it is very subjective on mood and what game I am currently playing.

I must say I spent a lot more time with Risen and Two Worlds II then I expected, but neither is the best games on the list, I was just positively surprised by both not expecting much from them.

You should have just ended the list there:)

I will probably enjoy many of those great games when I get to them. For now though of the ones on the list I have played I would pick:

The Elder Scrolls III: Morrowind

I spent over 100 hours in that game and eventually took over the world, killed all the guards and raided the Vivec treasury of all its loot and took it back to my home in Balmora which was there thanks to an excellent Morrowind residences mod. The armor sets were displayed there on Armor Mannikens thanks to another great Morrowind mod. On top of that I played through several very enjoyable, excellent adventure mods in the Morrowind world.

The funny thing is, I never did get around to actually completing the main storyline quest! I also never played either of the expansions. So with 100 very enjoyable hours into this game I still intend to return and probably spend another 100 hours doing the main storyline with a new character and also taking in the two expansions as well.

I just love the world of Morrowind. And talk about value for your gaming dollars. Wow.

Out of all of them Fallout new Vegas Ultimate
--its everything and its open and very fun i enjoyed the game the world and the story about as much as F3-GOTY there was so much to do and see and it was just fantastic

Im sort of bored with Skyrim already, made a mage, did the mage, main, brotherhood, thieves, and companion quests... it was short and sort of boring compared to oblivion and morrowind... but it was easy... there is just not enough content in Skyrim, not enough interesting quests... to keep me interested... it moved to fast, but i did log 200 hours on it since its release so.. well worth the money...

Morrowind is quite hard to track what to do, and its truly lacking any direction its the most open game on there where you have to "work" to get quests done. Nothing is handed to you and you really have to stay on your toes our you'll be lost in a few hours with a journal filled with a mess of things to do and go to with no direction at all. Hardest thing is figuring out what to do next, but that's half the fun i gather, never liked it myself... others hail it as the best of the best.

- Planescape: Torment
- Baldur's Gate 2 Complete
- Neverwinter Nights: Diamond Edition (for Hordes of the Underdark, for the editor and for all the amazing fan-made stuff)
- Arx Fatalis (not for the story, but for everything else)


Honorable Mentions (quite enjoyable, but not that dear to me):
Icewind Dale 2, Gothic, Divine Divinity, Arcanum

(Note: I might like but so far have never played any of the Fallout, Witcher, Dragon Age and Star Wars games.)

There are some really great games in that list. Trying to choose favorites is just some kind of punishment.

This.

Also, I like Shivering Isles but dislike Oblivion itself enough to not nominate TES IV as a whole.

I guess i should mention i really thoroughly enjoyed neverwinter nights 2... it was a great, very long game...

i think you should add Titans Quest: Immortal Throne on there.. long fun game.. spent more time playing that then any single game on the list...

Gotta be a tossup between IWD & BG.
Div Div comes a close 3rd.

My vote is for The Elder Scrolls III: Morrowind GOTY but there are a lot of games on that list I never played.

Honorable Mentions:
Fable: The Lost Chapters
Gothic 1 & 2
Star Wars: Knights of the Old Republic II: The Sith Lords

Fallout.

Although, having said that, I haven't played the majority of games on the list; regardless, Fallout 1 is a wonderful game =D

Mine will be The Elder Scrolls IV: Oblivion GOTY, followed up by Fallout 3 GOTY closely.

I have not got a hand on Skyrim yet, from what I heard and saw, my choice may change when I finally get a chance to try Skyrim.

And my other notable favourites not on the list are:
First two TES games
Might and Magic series

I'll vote KOTOR as that's the only one I've played out of that list. (And I've played KOTOR II)