Here's what I think:
Season 1: Great start to a great show. Many classics here, but some can turn boring with the time.
Season 2: Things start getting better by the second. Great humor, and great story lines, it still had it's blunders here and there.
Season 3: Perfection at it's maximum power. Each episode is hilarious and a high level of classic SpongeBob. Doubt it'll ever reach this good (I do hope so, tho)
Movie: Marvelous. I love each second of it, this is the movie I've seen the most time sin my life and definitely one of my all-around favorites. Season 4: The quality started decreasing, but it still had some great installments, altho, some of the show's greatest disgraces are in this season.
Season 5: SpongeBob goes way downhill with poor story lines and unfunny humor, it still had some great exceptions, and the specials were all good, IMO.
Season 6: Things are getting better. Some of the best plots can be found on this season (Not Normal, for example) the humor is clever, but there are many blunders.
Season 7: SpongeBob is back! I haven't seen one horrid episode yet, the humor is great and the plots are original and interesting, I can see things getting better and better for the show