The first three or four seasons are best romance in sitcom history imo. Not that there's much competition, really. Ben and Leslie are a close second, and they're better as an actual couple, but they don't have as many memorable and well-directed moments. They're really great at expressing a lot of emotions with just casual dialogue and expressions and timing without voicing their feelings outright. After they air all their hidden feelings and get together they're just not very interesting, though.
I'd say the first 8, 8 and a half seasons are pretty good, then it gets kinda awful in the middle. But they rectify it a little and course correct by the end of the series. ---