I played them all. Here is my critique:
Arena: Interesting promise, but it was dated on release. It came out a fully 2 years (ok 23 months) after Ultima VII and Ultima Underworld. Could have been better if they condensed the world, cities, dungeons. Just too little spread over too much empty. Not revered too much, but visited from time to time. There were sparks of brilliance in there, tho (music). Made wandering around interesting, and everyone owes it to themselves to fire up the GoG version at least once - seeing as its free (or was.)
Daggerfall: People look on it a wee bit with rose colored glasses, but it was buggier than Ultima 9 on release. People who played and finished it at the time hold it in high regard, folks who played it recently - well, may not see the magic. Excellent character creation, lots of content, lots under the hood but dated graphics even for 1996. Last one to require a manual, or at least last one you *should* read a manual.
Morrowind: Technical marvel at the time, still holds up. One of the most unique open worlds ever created. Hit the best balance on character creation of the series. Also (in)famous for it's quest instructions and directions (ie: West of where the rivier forks, North of the Ashlander camp...) Dialog system was bad. Still has a lot of life and mods today. This is the best TES Scrolls game for me.
Oblivion: Impressive graphics, big map, lots to do, but starting to diminish. You could tell the series was turning a corner. Fully voice by 4 people, better dialog system than Morrowind but starting to simplify everything for a wider audience. Dungeons really fell off (see: Ayleid Ruins) as well as the overall story. I liked the Daedra quests, but the the first Oblivion gate was interesting, the 97th was brutal.
Skyrim: The graphics are great, there is a lot to explore, and puzzles that any gamer can solve! The intro annoyed me and it was all downhill from there. Simplified everything. I'd rather play Arena than this. I did have some quality time modding this game though. Fell into the GTA IV trap for me. Let's make a simulator realistic to the point of boredom. I can see why folks like this tho, and maybe I'm wrong.