Here are my RPG recommendations:
2D, isometric view:
Fallout
Fallout 2
Baldur's Gate
Baldur's Gate: Tales of the Sword Coast
Baldur's Gate 2
Baldur's Gate 2: Throne of Bhaal
Icewind Dale
Icewind Dale: Heart of Winter
Icewind Dale 2
Planescape Torment
3D, first person view:
Morrowind (Can also be played in 3rd person view.)
3D, third person view:
Knights of the Old Republic
If you don't mind the 2D, isometric view and low resolutions, I'll specifically recommend the first Fallout, the first Baldur's Gate and Planescape Torment. These 3 are on my personal '10 Greatest Games of all Times' list, from any genre. (This year I have already replayed Fallout I & II plus Baldur's Gate 2 with expansion.)
For the game with gameplay closest to Neverwinter Nights, you should get Knights of the Old Republic, as the engine looks similar (except the camera is lower to the ground) and since it uses a similar system (based on Dungeons & Dragons 3rd Edition). I enjoyed KotOR so much that I played it through 3 times, back to back, averaging about 45 hours each time.
Have fun with your choice
