SGA is utter crud start to finish, the enemies are rubbish, most of the cast are significantly worse than SG-1, most of the stories are rehashes of SG-1 episodes.
The reason, IMHO, some people liked SGA more at one time is because the last few seasons of SG-1 are particularly terrible, while SGA either improved a little, or just seemed to improve in comparison to SG-1.
SGU is very different, there were some good episodes but ultimately it was full of too many poor cast members and far too many filler eps with very little going on. Season 2 was far stronger story wise but still entirely predictable.
Sg-1 is by miles the best, SGU second, SGA is just terrible. If I was going to rewatch anything, it would be SG-1 about up to SGA starting, post the attack on earth finale. Everything with the new guy and the Ori is dog****.