2002 is probably my favorite, as I consider that "my" OU Final Four, as I was relatively young in '88 and couldn't fully enjoy it. Not to mention, that team was put together EXACTLY how I believe a basketball team should be. But that is the obvious answer.
The 1999 team was a blast as well. That is the year we almost knocked off a really good Michigan State team to reach the Elite 8. That OU team wasn't all that gifted athletically, but they played great defense (60 ppg allowed), played as a team, and had an absolute warrior as a leader (Najera).
And probably the '90 team, as I remember being 12 years old and listening to Rick Fox (I've hated him ever since) beat us on that last second shot on a radio while out in my driveway shooting.