Suggestion: The Big Bang Theory's Irregulars

There is a group of actors who have recurring roles on America's most popular sitcom, The Big Bang Theory. I speak of actors who play roles on TBBT, not the many celebrities who have appeared in cameos. 

Which character most causes you to have an extra little giggle with they show up with Sheldon and the gang (five appearances minimum)? 

Althea 
Stuart Bloom 
Mary Cooper 
Dimitri 
Dr. Eric Gablehauser 
Beverly Hofstadter 
Priya Koothrappali 
Dr. V. M. Koothrappali 
Mrs. Koothrappali 
Barry Kripke 
Lucy 
Mike Rostenkowski 
Captain Sweatpants 
Emily Sweeney 
Leslie Winkle 
Mrs. Wolowitz 
Zack 

List: http://www.imdb.com/list/ls063117619