What is the best ES so far?

You can't just ask what the best TES is because the issue is subjective.

So, other than Morrowind and Oblivion, for me it's Redguard.

That doesn't give you much to choose from, and I doubt many people here have played both Arena and Daggerfall.

I'd say Daggerfall. Some would say Daggerfall is above Morrowind and Oblivion. Redguard is also good, though it's nothing like the rest of the series and the game mechanics are a little...off. But the story is great and the protagonist, Cyrus, is the best character in the series, in my opinion.

There is no "best" when talking about art. But my favorites:

Modded: Oblivion.
Vanilla: Morrowind.