Seasons 1-2, golden.
The Fairly Oddparents is a show that, today especially, could be easily ignored as another "hyperactive Nick cartoon". However, I recently ran into some of the older episodes of the show and re-discovered why I loved it so much when I was 10 years old (like Timmy Turner).
The characters are much more fun in the earlier episodes. Timmy uses wits more often than magic, his parents are just cute and clueless rather than outright brain-dead, and his friends have less flat personalities than just AJ the smart kid, Chester the poor white trash kid, and Trixie the rich bitch.
Cosmo and Wanda in particular are probably the most flanderized characters in the entire show, and the earlier episodes are shocking to watch because of it. Yes, Cosmo was always dense, but he actually came through with some great ideas now and then. Wanda was always more responsible, but she's hardly the nag everyone makes her out to be now. The best part about the series in the beginning was how Cosmo and Wanda were made to really love and care about each other. But in the latest live-action movie where Cosmo says Wanda will still have him when Timmy's gone, she shrieks as if death is the better choice.
Also suprising, Timmy's enemy in the beginning is Vicky! Yes, no Dark Laser, and much smaller doses of Crocker, Jorgen, etc. As a result, the episode plots were more based on real life situations (wanting to be older, winning competitions, trying to impress other people) and were less deranged than the new episodes about alien zoos and such. The first episodes focused a lot on actual consequences for the things that Timmy wished for, such as the video game in the first episode requiring tons of electricity. Remember the Season 2 episode where Mom and Dad thought Timmy shoplifted because they saw all the nice things he had been wishing for? Nowadays, Timmy can wish for volcanoes or fancy parties and no one's batting an eye.
At its core, Fairly Oddparents is a fun concept and the early episodes were good at balancing zany comedy with some geunine heart and thought. If you're interested in the show, Seasons 1 and 2 are great starters and the rest of the series stays fairly strong, probably until around Channel Chasers/School's Out. Then you're just watching bright colors, loud noises and jokes you can see coming as far away as China.