Of the tapings I've been to, Kunal Nayyar has been most likely to sign, followed by Simon Helberg, Johnny Galecki, Kaley Cuoco (she's only signed once), and Jim Parsons (he's never signed while I was there). Bill Prady and Chuch Lorre (executive producers) have always been happy to sign if someone waves them over.
I have all pictures of all of the autographs uploaded on my site here, but here are a few:

The booklet from the season 1 DVD set signed by (counter-clockwise from top left) Kunal Nayyar, Bill Prady, Johnny Galecki, Kaley Cuoco (in blue), Chuck Lorre, and Simon Helberg. My goal is to eventually get Jim Parsons to sign it, to complete the cast.

Counter-clockwise from top left - Bill Prady, Kunal Nayyar, Johnny Galecki, Chuck Lorre, and Simon Helberg. I'm also taking this one back with me each time, hoping to eventually get Jim and Kaley.

This is the program for "The Wheaton Recurrence" (at each taping they have a mini program for the audience with the episode title, and cast and guest star info), signed by Wil Wheaton, who was making his second guest appearance on the show.