I don't think Bob's personality varies between seasons. Take a look at season 7. There, we have episodes like Stuck in The Wringer, Growth Spout, Greasy Buffoons, Yours, Mine and Mine, Enchanted Tiki Dreams and Big Sister Sam. They may not be good episodes, but SpongeBob had perfect characterizations for each of those episodes. SpongeBob is pretty much like himself in all of them. Shown to be a perfectly jovial, positive thinking, optimistic and lenient being, but also capable to turn on negative emotions, like anger. And his intelligence are either forgivable and believable or actually quite shrewd-ish.
But then in episodes like A Pal For Gary, Tentacle Vision, A Day Without Tears, A Pal For Gary and A Pal For Gary, SpongeBob had from inaccurate to awful characterizations. Either he's way too stupid or just way too child-like.
Same goes for season 5 and season 6. SpongeBob's personality is perfectly demonstrated in episodes like The Splinter or Single Cell Anniversary. But then ripped apart by the writers who wrote Night Light or Squid's Visit.
All I can say is that SpongeBob's characterization gets torned rather rarely in seasons 8 and 9. Which is one of the reasons why it's getting a bit better. But to judge a character's performance by the progress of each seasons instead of episodes would be a bit hard. Since most of the time, a character's personality changes according to the tone of the episode. SpongeBob acted like a ::dolphin noise:: in Choir Boys because the episode's concept is to have Squidward's journey to the choir getting disturbed by SpongeBob. If it was about Squid and SB singing in a choir, SB wouldn't have his personality rather modified (for the worse, if I have to say myself).
So I would say SpongeBob's personality is...
