Arena: Tried it once, got out of starter dungeon. Realized I should just play Daggerfall instead. Stopped.
Daggerfall: 300+ hours. Played it like a sandbox. Never completed main quest, though got pretty far a couple times.
Morrowind: 1000+ hours. Completed main quest several times. Many started characters, several with individual goals. After a while it became a mod-testing sandbox. Then there's Tribunal. Man, Tribunal. I have a deep-seated hatred for it, being the first sighting of both the dreaded "requires key" and "levitation disabled".
Oblivion: 100ish hours of potato-faced adventure. Eh. Main quest twice, Shivering Isles once. Some mods, but was never very impressed with the title. This game made me buy a new video card (character creation was pure white screen with the menu on the left).
Skyrim: A little over 200 hours. Main quest once or twice, don't remember. Didn't buy DLC, may or may not in the future. Never very impressed with the title. The trying-too-hard-to-sound-epic music, the overly-burly nords (nix that, every race looked far too burly if male), and the far-over-done (in fantasy) dragons threw me off. Plus the fact that almost any other unexplored region would have been more of interest to me than draugr-infestation central. Then those quests. You want to be a bard? Ok, go kill some bandits. You will never play beautiful music, sera. Eh.
Battlespire: I wanted to like this game. Really, I did. The ability to create your character in such detail, even beyond Daggerfall's standards. The ability to go so far as to choose starting equipment. The art style, frost and fire daedra looking especially impressive. Then on top of all that, the ability to talk to the daedra? Yes! I wanted to play this. But there was too much trouble. The setup itself is an...exercise. Once ingame, there was just so much clunking about. The first level alone requires several hours, much of that being backtracking. I kept getting my [censored] handed to me, which wouldn't be so bad, except that I would just randomly die while walking down a hallway. No sound effect, nothing, just dead. Hmm. Maybe I'll try it again in the future, at a time I'm feeling particularly masochistic.
Redguard: Haven't played it. Don't have the game. It does look very interesting, though.