Blossom

Amy: The only person who signed my high school yearbook was my mother. “Dear Amy, Self-respect and a hymen are better than friends and fun. Love, Mom” 

Besides Harold's mother who is always off-screen screaming from the toilet while "during her business", as Howard says, Sheldon's girlfriend/non-girlfriend Amy Farrah Fowler is my favorite side character for obvious reasons: I've always had a crush on Blossom, and she is basically the female version of Sheldon, whom I have officially placed in the pantheon behind only Archie Bunker and George Costanza.  Everything she says is funny, and HERE'S a list of her Top 10 lines from a single episode, and they're all killer.  Actually, they've left off another great line, when she says she'll "give Leonard the night" to dream that she's into him.

My fave is from another episode, when she says "A gentleman at my door and a bosom buddy in my trundlebed?  I wish I could go back and tell 12 year-old me it really DOES get better!"