I find it hard to name the unfunniest comedies, because the really bad ones are probably the ones I quickly turned off and have long forgotten about. Also, I don't like to judge a sitcom until I've watched at least a few episodes of it, because it often takes a while to adjust to the humour. I couldn't make it through more than ten minutes of Mrs Browns Boys, because it's just not my cup of tea. But I can't call it the unfunniest sitcom when I've only seen ten minutes of it.
So of the shows I've properly watched, I guess I'd say the Birds Of A Feather reboot. I used to love that show when I was a kid, but I thought it was pretty bad when it came back. It was still watchable, thanks to Pauline Quirke and Lesley Joseph, but as a comedy it wasn't good and it was very dated.