I've played Morrowind, Oblivion, and Skyrim. Of them all, not counting graphics, Oblivion is, for me, the best of the lot. Finished Skyrim and have returned to playing Oblivion. As per the "essential", you might want to try the looking at the top 100 mods. Depends on what you are looking for. Some people really get into the immersive and landscape mods. Thanks to Wrye Bash, I am running 302 mods, batched down to 248.