I think the longest quest is the trying to collect all those aylied statues from different ruins. But if we're talking about Quests where your basically in the smae place and have the same goal all the way throught that quest, then I believe the very last mission of the theieves guild is the longest, where you have to steal the Elder Scroll. The very first time i did that quest it took me forever. But the reward was well worth it.
Bethesda makes the best games ever. The Elder Scrolls games rock harder then any other game ever. I like Heavy Metal and Death Metal. My favorite band is DevilDriver.