I've got a fair few autographs: Jewel Staite, Adrienne Wilkinson, Mary Tamm, Tom Baker, Nicholas Brendon, Greg Lutz, James Leary, Neill Cameron, Nathan Fillion.
My favourite and most prized though is James Marsters (Spike from "Buffy"). I only got it a couple of months ago. That man is an angel. He was supposed to be in England for two conventions, but he severely hurt his back on set, and was forced to move his flight. He managed to be there for the second convention (the one I was at!). Despite being in utter agony (he was wearing morphine patches on his arms and was lying down for all the autographs), he still managed to do a photoshoot with everyone who'd paid for one (me included!!!!). He was friendly, did several Q&A type panels, talked for a little bit to everyone who came up to him, and never once complained. He even apologised profusely to anyone who'd gone to the first convention and hadn't been able to see him.
Jewel Staite was also a total sweetie. I saw her at a convention in 2007, and have nothing but good things to say about her. From the start of the day, tickets to join her queue were going like hotcakes. When she saw this, she politely asked a event helper to fetch her a sandwich to eat at her desk instead of the 30/60 minute break the other celebs were taking for lunch. I saw her almost at the end of the day (after almost eight hours straight signing), and she still had a huge smile on her face. She apologised profusely for the queues, asked me if I'd managed to meet all the celebrities I had wanted to see (yes), complimented me on my t-shirt (Black with home-transferred glittery "Shiny!" on it) answered my question (which did she prefer doing Firefly, the series, or Serenity, the film - answer was "Firefly") and signed my photo:
Love from Kaylee
Made my day, and I honestly didn't mind waiting six hours to join her queue, then standing in a queue for an hour to see her.