OneFiercePuppy: Do you count DLCs or expansions in the tally? If you do, I'd probably have to say TES: Morrowind by a long shot. Just exploring the main game and doing the plot-related quests to completion should take you 80-100 hours. Add in the Solstheim and Mournhold expansions and you're way over 100 hours without ever going off an a truly unrelated sidequest.
With the official expansions, Fallout: New Vegas is also a very long campaign. And at least two of those expansions are really, really good. I think F:NV probably takes over 100 hours with the 4 official DLCs.
dtgreene: Actually, if you ignore side quests, Morrowind is not that long. A speedrun of that game (all main quests category) takes less than 30 minutes (maybe a bit longer in a marathon setting), so a casual run of the game would probably not take more than maybe a dozen hours. (Remember, this is ignoring everything but the main quest.)
Dragon Quest VII, on the other hand, is a lot longer; a typical estimate for that game is 80 to 120 hours, and even a speedrun takes more than 15 hours! (Based on the PSX version) Then again, DQ7 puts almost the entire game into its main quest, while Morrowind puts it in sidequests.
I thought about mentioning DQ VII, but then, most of the second act of the game is one sidequest after another that are connected to the main story line in some little way. I'm not sure that all the fragment gathering and island hopping actually would count as the main storyline and quest or more like a ton of little quests and stories in and of themselves.