Excluding games like Oblivion, WoW, Fallout (well, I never bothered with FO much but still) where the chances of faffing about are significantly higher than doing the main story: Probably XCOM: Enemy Unknown.
I actually failed my first time round, so I essentially spent over 40 hours to defeat the final mission, and it was so kick arse that I bought the expansion and put it another 15 odd hours, before replaying it twice more on the original version. Still, 40 hours seems about right. If I included WoW I guess it would be over 100 hours 'cause level 70 in TBC, but like I said, nope.