Definitely Charmed cast members. Thanks to LFCC and Collectormania, I have had the priviledge of meeting Brian Krause (twice!), Shannen Doherty and Holly Marie Combs.
It would be amazing if Alyssa Milano or Rose McGowan could be guests at a future event. Another list topper for me would be Julian McMahon too
Also, Supernatural cast members would be amazing. I met Misha Collins at Collectormania and it made my year! I'd love to see him again. And, on behalf of my sister, I would have to suggest Jensen Ackles (and Jared Padalecki for good measure) too.
And, finally, since I'm also a massive Doctor Who geek... my top picks would be Arthur Darvill, Karen Gillan, Louise Jameson and Gareth David Lloyd. I'm leaving out the Doctors for now, because the surviving Classics will be busy with Big Finish and the New Who Doctors will get mobbed! Haha