This thread is to find out which one you liked most. Please feel free to tell me why you liked it so much. This is not a thread to find out which is the best, because my favorite may not be someone else's favorite and vice versa.
As for me, I would have to say Oblivion. I came into the series late and played Oblivion before playing Morrowind. If I had started with Morrowind, I probably would have enjoyed that better, but I was spoiled with Oblivion and it was difficult for me to go back in technology.
Oblivion was there for me during a rough patch in my life. I didn't understand it at first, and even got frustrated with it and put it down for a month or so. But I went back in and played it extensively, over 3,000 hours worth at least.
I also enjoy Oblivion because it gives you a small taste of every region (except perhaps Morrowind) right on one map in good quality definition.
The spells were great, giving you a huge range and the ability to customize your own.
Oblivion still holds my favorite dungeon template -- Oblivion itself. Those hellish nightmares of dungeons reminds me of my days of playing the retro Diablo games.
I like Oblivon's pick-up-and-play pacing. Within seconds you could be adventuring and the load times are quick. The looting system is much faster than in Skyrim.