2001 gets props for getting to a bowl (with a brutal schedule) and holding MU and ku to w/o a TD for what seemed to be the 15th consecutive year. However, they were way more talented than 6-6, and losing 4 straight big 12 games when you had gone 34-6 the past 40 was just plain weird.
2004 gets my vote because of how easy the schedule was, and going 4-7 is just awful no matter the season. Seriously..4-7. They also just blew a bunch of games with terrible special teams, and that defense was depressing. the team was also coming off a conference title, so there must have been more talent.
2005 was pretty excusable. extremely young team.