2008 and it's not even close. Best game I've ever been to live (2005 Michigan Rose Bowl was a close second; I unfortunately was not able to attend the 2006 Rose Bowl vs USC).
As others have mentioned, both teams were ranked top 5 but, most importantly, OU was ranked #1. There were many ON THIS VERY BOARD who were acting like gaping vaginas before this game, talking about how we were going to get killed. Acting like OU was the best team they had ever seen and we had no chance. In fairness to those pussies, however, we had just come off a shaky 2007 season and it was still relatively early into our glorious 2008 run--we did not truly know yet at that point just how good we were. As we soon saw after that OU game though, 2008 would be our best team since 2005. We were even better in 2008 than we were in 2009--yes, we had the wrong team play for the NCG (it was nothing short of robbery and fuckery that our 2008 team did not get to play for the title that year against Florida).
As a result of the 2008 OU game, we obtained the #1 ranking for ourselves. The very next week, we continued down a brutal stretch of playing four top 10 teams in a row (OU, Mizzou, OkSt, and Tech). We held onto our #1 ranking until that horrible November night in Lubbock that would have been a non-issue (despite adversity) if Blake fucking Gideon just catches the easiest INT of all time. We would not lose again that year, but sadly Colt and Tebow would never meet each other on the field.
That said, fuck OU. We're coming for their ass this Saturday.