The Team Galactic episodes were spread throughout the entire series but if you watched them all in a row, they are really enjoyable. The conclusion was unexpected, especially since some certain events never occurred in any other episode. Team Aqua/Magma weren't as well done as Galactic but it gets interesting for a couple episodes once they obtain Groudon and Kyogre.
Sinnoh is arguably Ash's best season for development. He would have had a good chance at winning the league if it weren't for Tobias and his hax (most annoying league character ever, along with Cameron :P). Hoenn was good but there were so many boring fillers in between significant episodes (except Battle Frontier, which was full of great episodes). At least with DP, there weren't as many fillers in between (and there were more significant episodes). BW was a lot like OS but with a lot less humor. Little character development and a lot of having fun instead of serious training.