The venerable Christine Baransky as Leonard's mother, particularly her first appearance, where she deadpans a slice and dice matter of fact, psychiatric analysis of Raj's and Howard's "ersatz homosexual marriage to satisfy your need for intimacy," is one of the funniest things ever.
http://www.youtube.com/watch?v=qRPWluNgzRA
And I love the Amy Farah Fowler character, particularly since the actress playing the part really is a neuroscientist in real life after being a child actress. Nice touch.
Cowperson