MLB: Cincinnati Reds. No other team comes close. If the Reds are out of contention, I tend to then root for whatever team has some players I like. The Red Sox have usually wound up being my number two team.
NFL: Cincinnati Bengals and Oakland Raiders. When I first began following pro football, Cincinnati didn't have a pro team. I loved the Raiders offense and I've been a fan of both teams for more than 30 years. Except for a brief period in the mid 1970s and again in the early 1980s, the two teams have never been good at the same time, so a conflict in allegiance rarely has occurred. I will admit to pulling for the Raiders over the Bengals when they met in the 1975 AFC playoffs. I was a huge fan of the Stabler-Biletnikoff-Upshaw-Hendricks-Blanda-Shell-Madden Raiders teams. I also like the Green Bay Packers.
NBA: Nobody. I was a fan of the Lakers in the early 1970s when they had Wilt and West, but I haven't cared all that much since.
College football: 1. Marshall; 2. Alabama; 3. Penn State
College basketball: 1. Marshall; 2. Duke (my udergraduate and law school respectively)
Auto racing: I tend to root for whoever drives for Team Penske in open wheel racing solely because I was a fan of Rick Mears.