Takahashi Hiroshi's series were never my all time favorites, but I also don't like to include series I've worked on in those sorts of lists to begin with since they're all special to me. But Bad Boys wormed its way up to the top even before I started translating it myself, and working on it is only making me more aware of just how much I love it.
And yes, Kippo is the last in the line of series set in the Bad Boys universe. It features all of the Bad Boys characters are 40 year old parents, as well as characters from later series. It takes place in the modern day.
There's also Bad Boys Glare, set a few years after Bad Boys, and Megami no Oni, set in the late 70s/early 80s (so about 10 years before Bad Boys).