14th season, first episode. It was the one where they go on a road trip to Romania. At the end, when they were arguing about which car was the best between the Ferrari California, Aston Martin DBS and Lamborghini Gallardo, Clarkson says, looking at the 2 other presenters, "Look guys, the two people whose opinion on all matters motoring I respect the most" then the other two get a hopeful look on their faces, thinking he was referring to them, but then Clarkson continues on by saying "Tiff Needell and The Stig" the other two then look completely disillusioned "say that the DBS is the best car in the world right now".
I'm sure he must have said it in another instance, but that's the one I remember.