I think on Top Gear, Clarkson is the funniest (although a lot of it is obviously scripted to let him make his jokes).
But outside of Top Gear, Hammond is my favourite, because he has some really great programmes out there.
He was really funny on Brainiac: Science Abuse, and also on Total Wipeout.
And then he's narrated or hosted a number of documentary programmes, on a number of topics, from wild life to engineering, to geography to physics and more.