The Keys to the Alhambra stands out to me as one of the worst episodes possible.
Yes, same here. It didn't have anything extraordinarily memorable, just extraordinarily ugly.
I think the worst episode would be one where the following occur:
--A Moat that's really long because the players are poor contestants. Long moats are normally okay, but when the players obviously shouldn't be on the show, it looks bad.
--A Steps of Knowledge where the two winning teams sweep the steps, except for one correct guess by either of the two others late in the round.
--A Temple Games that is horrifically one-sided, has its outcome based on luck, or has a game like the common two-person pulley game of the first season where the game itself doesn't lend itself well to competition.
--And a Temple Run where one knows the players will lose after the first 20 seconds.
I'm not sure which episodes came closest to that, but I'd pick whatever ones did as the absolute worst.