2- it must be somebody who was only rarely on the series and never a regular (i.e. Don Knotts’ guest appearances after his departure on Andy Griffith don’t count and neither does Abraham BenRubi who though he’s always listed as a guest star is on almost every episode of ER).
My votes:
Maurice, Aunt Clara, Dr. Bombay and Uncle Arthur on Bewitched- they were all great, in that order. I’d wake up at 6 a.m. when I was a kid if the TV Guide said Maurice was going to appear on the morning rerun.
Carol Burnett’s crazy librarian character on Here’s Lucy. The show itself was bad, but her comic genius made her two episodes sparkle, especially when she goes from shy and bookish to crotch flossing while singing Hard Hearted Hannah (the Vamp of Savannah, G.A.)
Adam and Eve on Northern Exposure, but particularly Eve. Finding out she was a Christian Scientist (Adam: “You’re the damned poster child for the American Pharmaceutical Association!”) was one of the best surprises of all time.
Charlize Theron on Arrested Development for reasons already gone into on other threads.
Gwen on Angel, a beautiful woman clad in skintight black and red latex, she was a highly-skilled thief with electrical powers, who couldn’t touch anyone (or she would shock and possibly kill them). Obviously the writers had X-Men characters Rogue and Gambit in mind when they created her, but I liked her a lot from her two or three appearances in Angel season 4, and wish she had showed up to help the heroes at the final battle in the series finale.
For Angel characters who appeared more frequently, I liked the evil lawyers Lindsey McDonald (from seasons 1 and 5) and Lilah Morgan (from seasons 1-4) – they weren’t around all the time, but they were too much fun whenever they appeared.
Still in the Buffyverse, I ended up becoming a huge fan of Andrew, a misguided villain in season 6 of Buffy and an ally in season 7. He made two guest appearances in season 5 of Angel as well.
I always liked Sam Anderson (Holland Manners) on Angel. His portrayal of the amoral, coldly calculating Holland Manners is stark contrast to his current role of a devoted husband on Lost.