I had to leave Realm of the Hist outside this poll, because there can only be 20 possible answers.
Most of these we know very little about, so it's hard to judge.
While many of them are no doubt interesting, Apocrypha probably contains all the information in all the others, and so automatically wins. That said, Mephala's good at keeping secrets - but then Mora loves knowing other peoples' secrets.
I would think Apocrypha would be the most "interesting", because it has infinite amount of knowledge; therefore, its automatically more interesting because of all the interesting facts in that infinite amount of facts. If that made sense.
I wish we could vote for multiple ones just because I also want to vote for Moonshadow .___.
Eh, I don't like the phrasing of the question. We haven't really been exposed to many of the realms, except through reading.
From what we have seen in DLC's, I suppose I would say Sheogorath's realm is the most diverse and deep. So that would be the most interesting thus far.
I'm going to take this question as a different phrasing of, "What plane would you like more information on?". I guess I would say Vile's realm. I got pretty keen on it from the Greg Keyes novels. Would really be interested in checking it, and all the splinter realms, out.