Alright, here is my take.
In kind-of-an-order:
Wizardry 8
Ultima IV
Ultima V
Ultima VII
Deus Ex
System Shock 2
Fallout 1
Wizardry 7
Morrowind
Vampire : Bloodlines
You'll notice that there are no "official" Black Isle or Bioware items in my list. The truth is, I never got even close to finishing BG 1 or 2, lack of time and all that, and Icewind Dale, which I did finish, was very enjoyable but didn't make the cut.
I would love to have demonstrated my RPer pedigree by listing Planescape:Torment, but alas, I didn't own it till recently and haven't found the time.
I never played Arcanum to enough depth to make a decision on that either.
NWN is nowhere near my top 10; mainly due to the OC being rather bad, but the two expansions were very enjoyable, so those would definitely push it into my top 20. (BTW, anyone bought the premium-modules-on-a-cd that came out just recently?)
Daggerfall? Fuck I hated the combat in that game
(It has a lot of other stuff going for it though)
Hell, if it was "top 15" then I could have amazed you with my bad taste by shoving on Wizards and Warriors, Nox, and Black Crypt as well. :D