I'm going to go for Goodnight Sweetheart on this one. Underrated in the sense that it gets a lot of criticism and yet I think it is always a good watch and was quite an innovative idea for a sitcom rather than the average suburban sitcom that populated the screens around that time.
Lyndhurst played Gary Sparrow well and made his character plausible in what was, to all intents and purposes, an implausible situation and he was backed up well by the supporting cast, with Reg being quite an amusing Stooge and plot diversion and my attention was generally held all the while Emma Amos was on screen. Perhaps not underrated in terms of viewers as it ran for several series and attracted good ratings, but more in terms of critical perception and how it is regarded now in comparison to other comedies that spanned much of the 1990s.