Police Quest series from Sierra, up until PQ IV, including all collection releases.
I also included the first of the new spin-off series, Police Quest: SWAT, since it's a sort of transition title that didn't fit into either series. PQ1 to PQ4 were 'adventure' style games, while SWAT 2 and onwards are RTS tactical games. The first SWAT title was an oddball 'interactive movie' that failed to win any fans.
For the continuation spin-off SWAT series, visit this list: SWAT series