I think Morrowind is great. The immersion in the game world is really what does it for me - the size, the freeform ability to make any class, the massive number of quests, guilds, and factions.
Also, I love the editor. Just about every time I play, I tweak something in the editor first. Currently I'm running on 12 skill increases instead of 10 per level, and a slower skill progression in all skills, especially combat skills, because I found that leveling was a bit too fast and I would be a super-character long before I finished the main quest if I did all of the side-quests that I like to do. So I slowed it down a bit. Also, the two mods that change the NPCs to make them more individualized are great.
Of couse, I also am looking forward to NWN a great deal. But MW is great too.
If NWN is as good in multiplayer as MW is in single player, I'll be very, very happy.