The Anniversary, closely followed by Basil the Rat.
The Anniversary is a very flimsy premise to hang an episode on and as a result is drawn out, unbelievable and not very funny.
"Worthington, we're being attacked by giant bats!"

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The Anniversary is definitely the weakest, but there is no "worst" episode of Fawlty Towers.
Every episode is great.

I love all episodes but I also think that The Anniversary ist among the least good. The wohle scenario is just a bit too over the top, with Polly dressing up as Sybil and no one noticing the whole time, and Basil's and Polly's behavior was also a bit too silly. The simplest thing would have been to simply tell the truth and wait for Sybil to come back, but instead they get tangled up in stupid lies. I know that Basil often behaves stupidly and clumsily and loves to tell lies to cover something up, but I think even he wouldn't behave so stupid in such a situation. Other not so good episodes imho include A Touch of Class, The Wedding Night and Communication Problems. They all are far from bad, but just not as good and memorable as the really outstanding episodes like The Germans, The Hotel Inspectors, Basil The Rat, The Kipper and The Corpse, and my personal favorite, The Psychiatrist.

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Waldorf Salad is the only one I sometimes miss out when I rewatch them, as like many others, I find Bruce Boa's character way too annoying. The Anniversary is probably my favourite episode although I love them all except for the aforementioned W.S.

I know! the absolute best scene is him 'checking the walls' on the window and then reaching too high
ahhh.!
Oh God. Fortune
vomits
on my eiderdown once more.

^ this. Lots of classic farce in The Psychiatrist..
"Worthington, we're being attacked by giant bats!"

You call The Psychiatrist dull? This had so many great, memorable scenes, more than any other episode. E.g. the psychiatrist asking Basil How often do you mange it, Basil accidentally touching the girl, Basil standing on the ladder looking in the wrong room, Mauel saying "He try to see girl", etc.. To me, it's my personal favourite episode.

Not the worst, but
The Psychiatrist
was the dullest episode of the 12.

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I was never as keen on The Wedding party, and The Anniversary, and Gourmet night episodes.

The Wedding Party is my least favourite, seems a little forced and less funny than the others. Can't believe Waldorf Salad is rated so low by so many (perhaps not one of the best but surely it's somewhere in the middle?) or that some don't much like The Builders. I like The Anniversary, the only negative is that there's less of Prunella Scales than in the other episodes.

Actually, I don't have a least favorite. Cleese was always the maniac, and I'm
surprised that he didn't have a heart attack doing Basil. Everyone was great,
and I never tire of the series.

The Anniversary.

I'd have to agree with The Anniversary and The Builders - neither was really that good.

IMO The Anniversary is the weakest episode!

A Touch of Class, for me. The first episode and the show clearly hadn't found its feet just yet. Not that it doesn't have its moments, but compared to the non-stop laugh-riots that were to follow its fairly weak.

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My least favorite is also the anniversarylargely because it shows Basil in a sympathetic light in trying to do something considerate for his wife. While commendable, that really doesn't fit into the internal logic of the series.

For me, "The Builders" and "The Wedding Party" have always seemed inferior to the other ten episodes which outshine these two. Both are plot-less and one-joke farces.
Out of the two, I suppose I'd say "The Wedding Party" was the weakest - the only 'funny' bit I remember is Basil's "Oh, what a terrible dream" excuse (and even then, as Clesse admits in his commentary, it should be obvious to Basil that its Sybil outside the door and not the French lady). As for "The Builders", I think it might be marginally better, but I find it rather juvenille compared to other episodes.

No such thing.
They who give up liberty to
obtain a temporary safety deserve
neither liberty or safety

You know, it's kinda ironic but the Anniversary is one of my absolute favorites. It's up there with Basil the Rat and Gourmet Night.
But my least favorite was Communications Problems. I find the episode generally lacking in humor and I didn't like the ending, it was unfair to Basil and it wasn't because of Basil's doing. In most of the episodes of Fawlty Towers, when Basil would lose out, it was funny because it was of his own doing. I mean, if you look at the Builders, had he just listened to Cybil and hired Stubbs and not O'Reilly, everything would have been fine. In Gourmet Night, there are several moments where Basil could have avoided trouble but one is where Manuel comes to see him to tell him about the fight that has developed between him and Kurt. If Basil had simply listened to Manuel instead of ignorning him, he might have been able to stop the crisis. I could go on. But in Communications Problems, he's harmed by things largely out of his control and I didn't like that. Plus the whole thing with a deaf woman, I just didn't like it.

Mine is The Waldorf Salad but I still liked it.
"Life after death is as improbable as sex after marriage"- Madeline Kahn(CLUE, 1985)