In order of interest for watching on TV:
-Any form of Superbike/Supersport racing(World, British, AMA, etc.)
-WRC.
-Some forms of street racing.
-125cc class, MotoGP.
-V8 Supercars.
-250cc class, MotoGP.
-MotoGP.
-D1GP.
-Touring cars.
-Openwheel racing.
Hate to watch on TV:
-Oval racing.
-Drag racing.
My reasoning:
The closest racing is to be had only on closely-matched machines. MotoGP is fun to watch, but was frustrating this year for much of the year if you exclude the rounds with Melandri, Rossi, Capirossi, etc. fighting it out. (Rossi didn't win the title, but Hayden didn't deserve it after all of Rossi's bike trouble). I like close car racing, but close bike racing is 100x more interesting.
WRC is fun because they go at insane speeds off-road that I wouldn't even dare on pavement. F1 is similar in that the cars are never close to eachother
Moral issues aside, I find some street racing is fascinating to watch.
Oval racing and Drag racing CAN be fun to watch if you actually go to the events, but they're plain dull if you watch them on TV.