Morrowind, without a doubt, but this is perhaps because it was my first experience of a Sandbox game and, back then, I really enjoyed games without a plot. Additionally, Morrowind appealed to my imagination and I created my own story! My first character was a Dark Elf who was forever searching for her fellow prisoner, a male Dark Elf she had fallen in love with on the ship. My second character, an Argonian, was a freedom fighter who freed prisoners, killed those who wished to oppress others, and made a clean getaway by diving into the nearest lake and swimming to where no man, elf of Khajit could touch her.
After Morrowind, I really hoped to enjoy Oblivion; the opening was highly effective and exciting. Unfortunately, I hated the Devil gates or whatever they were! Having to complete these, I wasn't interested in progressing through the story.
Skyrim promised a lot, but the landscape was far too normal for my liking. I walk in the Lake District often, which is not QUITE as varied, but still breathtaking, exciting and picturesque. I prefer the real experience! The gameplay also grew old quickly for me. I have never found satisfaction in levelling up, in creating strong tanks that can bulldoze through dungeons! I prefer strategic, turn-based gameplay.