See, I haven't played any of the others. I'm not a video game person at all, and before Skyrim, I've only ever put time into sports games.
I would love to play Oblivion & Morrowind, but I'm not sure I'd be able to make the leap from Skyrim back to one of the older ones, just for the sake of graphics.
I love fantasy worlds and I cannot wait until they develop a game in which you can travel all throughout Tamriel freely. I know they'll have the technology to do it soon enough. Should be amazing when it comes about.
There's a massive 4 gig mod for Morrowind that makes it look really good, HD textures and all. It's well worth it if you're into fantasy worlds and everything, there's a ton more to do in it then there was in Skyrim.
Elite godly armor and weapons, levitation, jumping magick... all kinds of crazy things you can do in Morrowind that you can't do in any of the other games. There's a ton of random dungeons, underwater dungeons and so forth and most of the hard to reach ones have something that makes them worth going into.
I still play Morrowind occasionally even now.