What???!!! No, please! This question is causing my head to overheat...Well, I suppose my answer is - the sheer brilliance of the Rigsby character, the fact it was a proper sitcom, the fantastic set and Chappell's clever plotlines definitely get Rising Damp the vote over Perrin from me. Beckinsale also a big factor - ITV3 showed one of the very best RD episodes yesterday - Things That Go Bump In The Night, Beckinsale was nearly as good as Rossiter, and you can see why Rossiter and Barker rated him so highly. I'd still put Perrin as one of the 20 best Britcoms ever made tho.
People have mentioned the great Cinzano adds but who else saw the classic short film The Waterloo Bridge Handicap? (still not on any DVD compilation, it seems!) One of the funniest and cleverest little films I've seen and its star - the one and only L Rossiter (in a sort of Perrinesque role). Brilliant.