I voted for the 612 race to Verbier. I did not like the Mustang one--they said they waited for years to find the perfect situation to have another big race, but the car was silly and the film was too long. I always thought they should have done the 2004 race again but with a new Vanquish.
London race, GT-R, 1949, and original DB9 were also quite good. I think Jeremy admitted that they got a little carried away with the Veyron one.
Loved the Wembley to San Siro Mustang race, loved the truffle race, but my favorite was the Basil to Blackpool fuel economy race. That shouldn't have been as exciting as it was. The guys seemed to love their cars, and once Jeremy finally got into it, they really appeared to be having a great time.
Well, my favorite epic race (which I watched) was the Veyron vs Plane. But, if May could also fly in the Night, he'd have won it easily, that he had to land in France costed him the victory, but gained us more excitement about that race.
Veyron one didn't really work for me from a race pov, this whole "I'm not allowed to fly at night and we didn't anticipate this being a problem" is silly. Personally I like the SLR one, not necessarily because of the car but because you could see how sheer will triumphs massive exhaustion. I also very much liked the car vs train vs bike thing.