I'm going against the curve and opining that Bill at his worst was Hot Potato and The Joker's Wild. Cullen appeared lost and out of his element in both shows.
I thought he handled Hot Potato very well, even with the upstaging celebrities. The Joker's Wild, on the other hand, was definitely mishandled. Bill was already "past retirement" with Hot Potato... even someone associated with the show said that it didn't make sense to replace Jack Barry with someone whose health wasn't all that great. However, the powers that be didn't think Jim Peck was well known enough and wanted a very established name to host. Bill was fine on TJW, it's just that his slower pacing and lack of opportunities to joke around made the game overall seem slow. They got Jim to fill in for Bill at least once, and I'll bet they would've had Jim complete the run if Bill wasn't well enough to return.