MLB: Reds. I have been a Reds fan since 1966. No other team comes close to the Reds, but the Red Sox have been my favorite AL team ever since the Impossible Dream '67 team.
NFL: Raiders, Bengals, Packers. As a boy we could only get AFL games on TV and the Bengals had not yet been created, so I became a Raiders fan in '67. I loved the Madden era Raiders. I do also root for the Bengals. Reading "Instant Replay" as a boy helped make me a Packers fan. Rarely have more than one of these three teams been good at the same time.
NBA: I liked the early 70s Lakers of Wilt and West and haven't cared much since then.
NHL: No interest.
College: Having graduated from Marshall and from Duke, I root for those schools. The first college football games I ever saw on TV featured Alabama and I have rooted for the Tide ( and against Auburn) ever since.
High School: I realize no one else mentioned this grouping, but my sons attend/attended the same school as my wife and I, and Wayne High has won three state titles in football and two in softball since '06, and I do follow the Pioneers.
Golf: No interest. The last time I got interested in a golf tournament was the final round of the '86 Masters when Nicklaus turned back the clock and won.
Racing: I was a huge Rick Mears fan. I haven't cared nearly as much since he retired 20 years ago but I do root for Penske in Indycar because that was who Mears drove for.
Tennis: No interest.