My own favourites
Series 2 of Fawlty Towers - well worth the 4 year wait
Series 1 of I'm Alan Partridge
Series 2 of Porridge
Series 1 of WHT The Likely Lads
Series 8 of Steptoe & Son
Favourite series (seasons) of sitcoms
My gut instinct is to say series 1 of virtually any series because that's usually when I fall in love with a show. And, as with relationships, the early days are filled with excitement and discovery.
Series 4 of Seinfeld is probably the best series of any sitcom ever.
Season 4 of the American Office is very strong. A lot of those long running shows take a while to really hit their stride - Seinfeld being an excellent example.