In my humble view...
Kandi: She was hilarious and deserved way more than 17 episodes.
Jerome: How the hell was he only in 2 episodes??? He was hilarious!
Gordon: Funny character and a good friend to Charlie who only appeared a handful of times in five seasons (1, 3, 4, 6 & 8).
Herb: Even though he appeared more when together with Judith, he was still pretty underused.
Melissa: Loved her! She was so adorable.
Myra, Herb and Lindsey
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I couldn't stand Kandi, or Candi, or whatever her name was.
My vote goes for Teddy. I know Robert Wagner may not have gone along with being a regular, but he was fabulous in this. Too bad they changed the writing and made him a scam artist. His first character arc as a millionaire was better.
Would have loved to see more of Russell, specially after Charlie left the show.
Jonathan Banks's character was fun too, just a cameo but it would have been a good occasional character.
Gonna add another: Norman. He was the old rich guy who was married to the young hot tramp that Charlie was banging (actually upstairs while Norman knocked on the door looking for her). I know this really doesn't count because he was one and done, but he was fabulous in this.
The first time I watched this episode, my wife and I both thought he was really funny with fabulous timing and delivery. We looked him up during the show and were stunned to see it was Orson Bean, a person I never liked in anything when he was younger. But in this, as the bitter old but wiser man, he stole the show.