Hmm, now this is a tough one.
I think TES games are very hard to evaluate which one is the better, because each of these has at least one feat it excels, and many others it sucks at. As such:
The Best TES games:
1. Morrowind: best world building and overall sandbox freedom
1. Skyrim: most accessible and overall production values
1. Oblivion: best side quests
1. Daggerfall: most ambitious and overall pioneer charm
1. Redguard: best puzzles, story and overall boldness to break the mold
The Worst TES games:
-1: Morrowind: bad physical combat, bugs
-1: Skyrim: totally bland world building, bugs
-1: Oblivion: botched rpg mechanism, bugs
-1: Daggerfall: very repetitive, bugs
-1: Redguard: clunky controls, bugs
Arena and Battlespire are omitted, because Arena was uninteresting to me at the time (obsessed Ultima fanboy here), and Battlespire was so buggy it never ran on my PC more than 10 minutes. Also, TES:O obviously not on the list, as I still believe humanity is not yet prepared for playing MMOs proper.
So what's the verdict? I love'em all, bugs and quirks aside.
Overall, I have ALL these games installed in some ways, and play them occasionally as my mood dictates.