Easily Daggerfall without doubt.
Yup, and anyone voting for something else hasn't played it.
Dis/advantages > lousy birthsigns
35 skills > 21 skills
Ability to alter attributes > inability
Ability to alter skills > inability
Abilty to alter social group dispositions > whole damn things removed from later games
Abilty to choose how fast character levels > leveling too fast no matter what you do
Ability to play without magic > some spells are forced on you no matter what
Only thing positive I can think of is the face generator, which has nothing to do with roleplaying elements. And to be honest, the face gen in Battlespire is almost as good as in TES4. Uncanny valley of Cyrodiil is also a big minus for the more 'realistic' graphics.
Plus Morrowind added a couple of good skills, even if it removed much more.
Edit:
Ability to choose hitpoint gain per level > a stupid multiplier system that forces you to powergame