I'm writing this by memory so they may be a bit off:
JC while revewing the MG-SV:
"Zo, ze Brrritish built a carr wiz spit und kleenex! Zey vill be crrushed."
JC while revewing the CTS-V:
"This car was planned by someone in America that came across a very rare book, an Atlas! And in that book they discovered a lot of places that weren't America, they saw that one of those places was Uoooroope, and in Uoooroope they saw Ghermany, and in Ghermany they found the N?rburgring!"
James May while driving the A-Class Diesel with the owner in the back:
JM:"May I ask why you choose the Diesel version?"
Owner:"Why not the Diesel?"
JM:"Because it's slow, noisy, rough, rumbly and it shows to everyone else that you're a bit tight."
While doing the Porsche Challenge and JC discovers a hole in his 928 fuel tank:
JC:"Well that explains the poor fuel consumption!"
RH:"And the trail of fire."
JC does a race in a Noble against Hammond in a Morgan for a lap, and bets that he can read a book before Hammond reaches the finish line (it was a children's book):
RH:"That's not a real book!"
JC:"It is a very good book, it's called A Very Hungry Caterpiler, and it comes with a caterpiler plush toy!"
-tosses it into the Morgan
JC:Watch out it doesn't eat your chassis."
JC while revewing the Rolls Royce Phantom:
"You open the door and there is a hole to put your umbrella... that's nothing special, the Skoda Superb has this... It's the only superb thing about it."
JC trying to do doughnuts in a Smart Fortwo:
"Technicaly any rwd is capable of doing this!"
- tries it, fails and gets out of the car
"I guess you need more than 1 hp."