Personally, as a huge Buffy fan I would love to meet Sarah Michelle Gellar, Michelle Trachtenberg and Anthony Head. But also the casts of the TV shows:
PLEBS: Tom Rosenthal, Joel Fry, Ryan Sampson & Tom Basden.
Spaced: Simon Pegg, Jessica Hynes, Nick Frost, Julia Deakin, Mark Heap, Peter Seafinowicz, Bill Bailey & MICHAEL SMILEY!
Hyperdrive: Nick Frost, Kevin Eldon, Miranda Heart, Dan Antopolski, Stephen Evens, Paterson Joseph...
Also the cast of Green Wing would be LIT!!!