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.
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.