That depends on your budget.
Zooms..... Sigma 150-500mm or 50-500mm ideal for wildlife, perfect and the cheapest way to 500mm, personally too slow for motorsport. Sigma 120-300mm f2.8 OS with TC can be quite an adaptable lens, good reviews for motorsport, but top budget. Sigma 100-300mm f4 (if you can find one), fantastic lens, sharp reasonably fast, work ok with 1.4x TC. Canon 100-400mm, still going strong and if you get a good copy, is a great lens.
Primes..... Canon 300mm f4 IS (non IS used), great all rounder and works well with the 1.4x TC, 400mm f5.6 ideal for wildlife, sigma 300mm f2.8 all around the low 4 figure bracket
Bottom of the pile in terms of cost sigma 120-400mm, slightly faster than its bigger brothers. So it really depends on your budget?