SG-1's first season is really weak, though there's some great episodes throughout it and by the end it's certainly picking up steam. Season 2 is definitely when it got good and doesn't disappoint.
Seasons 2-5 are generally considered among fandom in general to be the show's golden years. I personally think the show stayed good up to season 8, though seasons 6-10 get mixed opinions among fandom. And while I don't think seasons 9 and 10 are as good as the first 8, I can't call them bad seasons and they are worth watching all the same.