My favorite is The Elder Scrolls II: Daggerfall just because of the overwhelming amount of content in the game.
I didn't know this until recently but there are over 750,000 NPCs, 15,000 towns, and 487,000 square kilometers to explore in the game. It supposed to take two real life weeks to walk from one end of the game world to the other. I myself have never attempted this so I don't know for sure.
To be honest I haven't played Daggerfall as much as I have played Morrowind or Oblivion since I only recently downloaded the game. Only about 70 hours in, but once I got used to the controls I haven't been able to stop playing it.
By the way, thats more content than Morrowind and Oblivion put together and then multiplied by 100..
For the sake of simplicity, may I ask why the expansions have been separated from the games they expand? They're not separate games, just additions to their respective games.
As for Daggerfall, it may have more of everything, technically, but in the most literal way, once you've seen one location or done one side quest, you've pretty much seen or done them all. I don't mean to sound like I'm bashing it, but you seem to have your expectations of the gameworld far too high.
My favorite is Oblivion, but that includes SI and KotN.