Morrowind is best Elder Scrolls game.
Morrowind was not fun to play, I enjoyed it. If that makes any sense.
I didn't like the game mechanics but I still liked most other things about it.
I'd have to say Skyrim is the best for me, I can't see any reason that I would decide to play oblivion or morrowind over skyrim.
The thing I like about owning these games on pc is that anything I don't like about the game I can generally change with mods.