Morrowind is my favourite.
I've played Skyrim for ~115 hours so far, completed Main Quest, Companions and College, then started second character and somewhat I lost interest for now, though I'll surely play it more someday.
I've played Oblivion, probably for no more than 250 hours since 2006, I've "beat" (like in completing MQ and all factions) the game once, then tried returning MANY times but couldn't get too far.
I've bought Morrowind in 2005, I'm playing it since then (I'm guessing well over 1000 hours on the counter), I've even got 3rd copy just yesterday, so I'm going to play even more. It's just the alien, mysterious setting, that I got first hooked to when I was 13 years old.