All right, got some more time, so I'll expand a bit on my choices.
Favs
Fallout.
The freedom, the great ending including the slides, the postapoc world, the great quests, violent encounters, top notch voice acting and the fun faces. This game did almost everything right and was a blast to play through.
Planescape: Torment
One of the best stories I've seen in a computer game and the best one in a crpg. The many varied ways to play thorugh the game, the locales, art direction and great npc's. This game was a great rpg, if you like a well told tale. I do and love it.
Good ones
Baldurs Gate 2
It was huge and seemed rather open at the start. The amount of quests, the class specific ones (keep, theatre) added to the replayabilty, a number of fun npc's, the great art direction and the fun main story about the Bhaalspawn all made this one of my fav games. It felt rather railroaded after act 2 and the combat isn't one of it's best points, but even then it's defenitely one of the best rpg's ever.
Arcanum
I absolutely loathed the combat, the dull and stupid dungeons and the unbalanced game mechanics. However the world was well crafted and fun to explore and filled with real roleplaying possibilities. A flawed gem, but brilliant nonetheless.
Fallout 2
Had almost everything that made fallout great and often improved on it. However it missed some of the atmosphere and the story just wasn't as interesting. Still, it ranks as one of the best in my book.
Honourable
Dark Sun
A great spiritual predecessor to Baldurs Gate 1. You start out with a death sentence in an arena and work your way up to become the leaders of a rebellion. Great gameworld, fun quests and a rather good overall story. Still highly recommended today.
Buck Rogers: Matrix Cubed
This game has everything that it's predecessor had, but improved on it. A fun corny sci-fi setting, cool combat and an entire solar system filled with quests to explore. Still worthy to play today, even without the nostalgia factor.
Ultima 7.2 Serpent Isle
Took me ages to get this bitch running, but it was pure joy when I finally managed. For the first time that I could explore a huge living breathing world.
Darklands
This game kept me locked in front of my monitor for ages. So many things to do, try out and see. Even on my last replay I got new encounters. It's aged nowadays, but for a fun romp through a medieval Germany where the old tales about witches and demons are true, get this.
NWN2: Mask of the betrayer.
It still has a shitty camera, over the top monty haul approach and rather uninteresting combat. It does however offer one of the best roleplaying experiences in ages. Get it, it's everything nwn2 should've been at the start.
Games I liked, but missed that certain thing to make them classics.
Starflight 2 (This game took away an entire summer of my youth. The huge galaxy, many races and great story kept me playing this)
Kotor (great start, but felt a bit boring from half way through.)
Kotor 2 (suburb, but then you get to the end and all the cut content)
Star Control 2 (rpg? never mind, it's a great open ended game)
Buck Rogers: Countdown to Doomsday (great sf game, but the sequel was better. Still the alien mission is one of my coolest gameplay experiences ever)
Ultima 7 (exploring Brittania was great. But I played 7.2 first, so it has the nostalgia factor)
Jade Empire (was a lot more fun than expected. Solid bioware rpg, nothing more and nothing less)
BG2: Throne of Bhaal (fun dungeon romp, though you need that pacth from one of the developers to get the most out of it)
Vampire: The masquerade (Another pure Troika game that just needed more time and polish to become great. The end kinda fucked it up for me)
Prelude to Darkness (it was a great roleplaying game for me. When it didn't crash every 5 minutes, that is)
Gothic 2 (it's hard, has a great breathing world to explore and looked stunning at the time. It's a bit railroaded at the end, but still a great game to play)
Avernum 4 (If you like exploration, you'll like this. It feels indie all over and the setting is getting a bit tired nowadays. Still, it's the best avernum yet and great fun to play)
EDIT Excnhanged Starflight for Dark Sun. Both are great games, but dark Sun is more of an rpg.