I've never really used any mods with the first Kotor save for one which unlocked a cut ending for the game. Playing as a female Reven and pursuing the romance with Carth, one of the possible endings was essentially allow yourself to die knowing that the lure of the dark side was too great and it was inevitable you'd fall once again. Very sentimental and dramatic in one of those tragic ending sort of ways, very cool!
Still I can understand why they cut it considering it meant Revan died at the end, which wouldn't have made Kotor 2 make much sense. But still, neat ending.
For Kotor 2 however the
Restored Content Mod is absolutely essential. So much so I'd say the game isn't worth replaying without it. That mod essentially takes everything Obsidian had meant to do but didn't have the time to complete, and finished it for them. And damn is it an improvement!