For me it would be Better Bodies and the many head replacers. Looking back at the hideousness of the original models I realize why I always played first person perspective (which is not the case any more)
Also Morrowind Comes Alive for a similar reason as Dragon_Wenches selection of Children of Morrowind. It breathes some life and unpredictabilty into the game.
( I couldn't Children of Morrowind to work - I might try it again in the future if only to be able to run round and bludgeon the little darlings to death
).