Start by trying to work out what focal length you need then look at the options.
Just like Phil has said above, I use a wide range of lengths depending on the shot I am after.... 10.5mm all the way up to a 500mm with a 1.7x TC on it (850mm). Mainly though its the "classic" motorsport combo of a 70-200 combined with a 300mm and with a 1.4x TC in my pocket.