In Oblivion we had Mehrunes Dagon's Deadlands, Sheogorath's Madhouse, Peryite's Pits and Boethiah's Attribution's Share. Peryite's Pits and Boethiah's Attribution's Share both look very similar to Mehrune's Dagon's plane in Oblivion, so I put them in the poll. We also had Clavicus's realm in Redguard, but I figured it could be redone.
I left out Mephala's Web because it is inaccessable to mortals, but I left Azura's moonshadow in because while it is so now, it has not always been inaccessable to mortals
This page describes the various planes of Oblivion: http://www.uesp.net/wiki/Lore%3a%50lanes_of_Oblivion
If you have not completed the Shivering Isles, then it may contain some spoilers.
For me it was a choice between Hermaeus Mora's Apocrypha, Vaermina's Quagmire and Hircine's Hunting Grounds.
I chose Hermaeus Mora's Apocrypha as it sounds the most interesting: "Apocrypha is Hermaeus Mora's realm. It is an endless library where all forbidden knowledge can be found. The books all have black covers with no titles, and the library is haunted by ghosts forever searching for knowledge."
As for the second question, I chose Clavicus's realm, because I have never played Redguard and it sounds like an interesting realm.