Worst theme - tie between Pyramid and TTD 90. Might lean more toward TTD, as Pyramid was just a rhythm track.
worst is a relative thing - I'll pick a few that left a bad taste -
Host - Red Rowe, nighttime Video Village (Narz was just as gabby in the early nighttime shows before it moved to daytime, as it was a new show and there was a lot to explain. But when Rowe took over..oh my God, Red..SHUT UP and move the game along!!!)
Format change: the second incarnation of Play The Percentages really blew. Three On A Match going from matching prizes to pictures on the board was weird.
Show - boy, a lot to choose from - Winning Streak (weak main game, fair bonus - dull, dull, dull), Blankety-Blanks, but big ones that come to mind are $1.98 Beauty Pageant, Three's A Crowd, and The Neighbors for their entertain through humiliation factor.