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.


People would probably complain anyway... "Too many trees!!" Also, imagine the lag you would have got back then... 