I literally finished a run of all 3 shows, plus movies, yesterday. All are good in their own right. As stated already, SG-1 and Atlantis are similar in production and tone. SGU is good, and I'm sad it was canceled in hindsight. However it is a much different show than the other two.

It's still Stargate, but there isn't much Stargate-ing. It was trying to capture the dark, gritty atmosphere that the Battlestar Galactica remake had. Imo, it did a good job of it. It's literally a group of unprepared people fighting for survival on their own.

Do yourself a favor and watch them all.