Who is Your Favorite Character Who...
...was only featured one time or, at most, a few times? This topic is meant to exclude reoccurring characters or cast members who were on regularly for a certain period. (Although if someone can't help themselves from an honorable mention like Percival Dalton AKA Isaac Cohen, I'll understand. )
My favorite is probably Isaac Silverman, the craftsman for whom Albert apprenticed. He was hilarious and full of wisdom. Killing him off in his one and only episode was so cliche, but it was done well with Albert finishing his coffin.