OK, in alphabetical order as they are all great for different reasons:
Ariel Atom (I'm not bandwagon jumping - it's been on my shortlist from the start.)
Mercedes SL55AMG (A Clarkson Classic, with a wonderful contrast between the start and the finish - sadly the version on TG Web doesn't include the bit where they swipe his keys to give it to the Stig.)
RR Phantom Drophead Coupe (Oasis of James May in a desert of Clarkson and Hammond during that particular series, and in my opinion probably Slow's best ever solo piece.)
Pardon the pun but I also toyed long and hard with the idea of the Carver, but decided not to once I discounted the hugely influential "WANT" factor.