Here's a list of 20 candidates - a few have been mentioned already - that I can think of:
Usain Bolt (maybe not in season 18, but nearer to the Olympics next year in London)
Michael Phelps (same as Bolt)
And of course...Michael Gambon!
Some have been on TG before, others haven't. Some are Brits, others not, but all would be great guests - and the majority would post good times - in my opinion.
P.S. - If you're going to have Angelina Jolie on, you may as well have her bring Brad Pitt with.
(I'd also quite like to see Chris Tremlett, but only because he's taller than Jeremy and it would amuse me to see Jeremy being the short one)
Bill Oddie (just to see him try and get back the money for all those speeding fines!)
Paul Merton and Ian Hislop
Christian O'Connell (because he is bound to be rubbish but hilarious!)
Johnny Depp (*drool*)
Alan Rickman (*even more drool*)
Flea (Red Hot Chili Peppers)
I'd be OK with Angelina (but am not particularly interested), but definitely not with Brad Pitt (yuk!)