Moshkito
Hi,
Not exactly a sitcom watcher, but if I have to choose one, I would say that "Fawlty Towers" was the one I liked the most.
So would I. Though I suspect it means something different to British people around my age, who can remember the kind of characters it portrays being around, than to a younger audience. Nowadays it sometimes gets accused of being racist, especially the character of The Major, but back when it was made that kind of casual, automatic racism was very common as was the class snobbery of e.g. Sybil Fawlty. Cleese and Booth weren't exactly popular in some quarters when they ridiculed either.
The same sort of thing applies to the Monty Python stuff. A lot of it only makes sense (to the extent any of it makes sense) if you're aware of the prejudices of the British middle-classes and the kind of regime found in English Public Schools (which means private and very expensive schools like Eton) in the 50s and 60s and some of the stuffier Oxford/Cambridge university colleges around the same time.
Edited to add - for a radio sitcom, try the BBC's "Cabin Pressure". Benedict Cumberbatch before he was famous, and very well written and acted. Parts of it compete with PG Wodehouse, which is high praise coming from me.