OS: Best traveling group, hilarious banter between Brock, Ash and Misty, Team Rocket at their best, Ash being a cocky brat and a noob trainer, Charizard being a dick.
AG: Ash showing signs of growing up, OMG why did Misty get replaced, unbearable little sibling, battles getting better, Ash's best achievement till date as a trainer, fanservice of seeing Kanto and reserves returning.
DP: The peak series (to many), best developed female companion, best rivalry in series resulting in the best battle seen in the whole anime series, Ash showing four regions worth of experience, terrible fillers, infamous 50 episodes gap between 7th and 8th gym battle, walking plot hax with Tobias, and the end of an era with Brock's departure for good.
BW: Pikachu's infamous loss to a starter Snivy, infamous reset of Ash, terrible companions, underdeveloped and weak Pokémon used by Ash, DEM galore, worst character seen yet, and even worse being the loss to him in the league, awful poor man version of Gary/Paul as a rival, desperate fanservice seen with Dawn, and especially Charizard's returns which served no purpose.
XY: Also considered the peak season of the anime. Ash gets the recognition and respect after 6 regions of traveling, most shoenen series of the anime. Ash at his peak as a trainer, the start of a ship that is closest to canon, great personal development for said female companion, amazing art style and animation for battles, Ash Greninja and Ash going neck to neck with a champion, salt galore after league loss, and amazing villain plot that trumps the game version by far.
SM: Again a total 180 from the previous series, slapstick and cringe faces galore, no more traveling, Ash apparently goes to school now, Ash and the overall art style looking weird as heck, too many companions, with half of them being deadweight, infamous for rushing through plots (like Akala, and AF), and too cartoony and goofy in tone.