I'd say the end of season 1 then it ran on high for the next few. Season 5 it started dying but I think with Jonus season 6 really kicks some life back into the show.
Then it's just downhill from there.
I've been enjoying it all the same. RDA O'Neill is absent for 99% of the time but I've found the Jaffa Nation, Ba'al as baddie, Replicators, Ori and Lucian Alliance elements all engaging + Mitchell and Vala(but I also love Farscape so possible bias there).