It's pretty average throughout, but season 1 is actually one of the better ones. IMO it starts rather well, goes a bit downhill and season 3 is rather bad/uneven, season 4 is the best, and season 5 has some strong episodes as well, then it really goes down and season 7 is the worst
How can you say it really goes down in season 6? There's no great difference between seasons 5 and 6 in my opinion and season six gives us "Survival Instinct", "Barge of the Dead", "Tinker, Tenor, Doctor, Spy", "One Small Step", "Pathfinder", "Blink of an Eye", "Memorial", "Child's Play", "Muse" and "Lifeline". There are a lot more excellent episodes in season six than season five.