I'm sad that I didn't participate in this. I didn't know it would be an official thing.
The list is good, though. I'd have voted basically the same way.
1. PS:T
2. BG2
3. FO2
4. FO
5. BG
6. Arcanum
7. Diablo 2
8. Bloodlines
9. Morrowind
10. Deus Ex
That's my loose judgement. I focussed mostly on how much time I've spent playing the games and amount of times replayed. Then threw in an enjoyability curve.
For instance, I've probably spent roughly 10x the amount of time on Diablo 2 than any other game, but the memories aren't as fond.
PS:T was a little short and had a tight path, but is just written so well. I read the text out loud when I play. Also, most of the exp in the game is achieved through dialogue. I loved that.
I've probably replayed BG2 the most times out of any of these games. I'd say over 15 replays. Not neccessarily completing the game each time, but playing it to almost the end. I've played it through to the end of Throne of Bhaal four times I think.
Fallout 2 beats Fallout narrowly. While Fallout is The standard for quality of an RPG, I've played FO2 many more times. It's a close race with BG2 in the replay department. Although, I almost never finish the game. I just loved the massive world. There are tons of towns and fun quests to do. The original Fallout is classic, but there wasn't as much to do, so I always ended up playing Fallout 2.
I put Baldur's Gate as 5th for kind of the same reason I put Fallout under Fallout 2. Baldur's Gate had a great world and a cool story, but BG2 just had way more to do. BG was no slouch, but it was mostly dungeon crawling and exploring. I like NPC interaction a lot and there wasn't that much compared to the above games.
I've only beaten Arcanum once, but I've played it a few times. If a certain quest bugs out on me and I can't do it I end up quitting. The world is awesome, though. The combat is really bad, but the NPC interaction is high and there's tons of places and quests. It's a buggy piece of Troika shit, but if you ignore that then it's great.
Diablo 2 got this rank because I've spent so much time playing it. It's really the most addictive game I've played. With a maphack and a nice bot you almost never want to quit. It being a multiplayer game, it doesn't have the appealing aspects that the above games have. Barely any dialogue, not much of a world to explore, not very many quests. It's just so fun collecting stuff and becoming uber leet, though.
Bloodlines goes here, because it's a buggy piece of Troika shit. I love it. I replayed it four or five times. The design was great. The quests were cool. The dialogue was very entertaining. It was kind of short and the story was a bit disappointing. The combat was frustrating, but sometimes fun. If only Troika had better programmers...
I've just been playing Morrowind again. It's a huge world with tons of quests. The scale world is just really awesome. The dialogue is horrible, the main quest is meh, the combat is also meh. It's really fun, though. I like becoming the highest rank in every faction. If you do it in the right order then you can join all of them and one of the three houses. Telvanni, Redoran, or Hlaalu. You can eventually get your own house and that's always cool. I tried to collect every set of armor and rare items to decorate my house. I have screenshots of it somewhere. It's seriously lacking in the dialogue department and that's the only major complaint I have. Dialogue is a tremendously important aspect of a game to me, though.
Deus Ex is really fun. I've replayed it a bunch of times. The whole part about it basically being an FPS makes it a little meh. It has experience points and stats which is really cool for any game to have. You have a lot of choices for the ending which is great. It's not much of an RPG, but I enjoyed it a lot.