Also, depending on what deck you want to build, each set tends to cater to a specific type or two for support:
XY
Stadiums for Darkness and Fairy
Some good staples like Sycamore, Professor's Letter, Shauna, Hard Charm, and Muscle Band
Flashfire
Fire support in Fiery Torch and Blacksmith
Splashable Pokemon like Kangaskhan-EX and Miltank
Staples like Lysandre, Pokémon Fan Club, Sacred Ash, and Ultra Ball
Furious Fists
Fighting support in Strong Energy, Korrina, Fighting Stadium, and various good Fighting types
Grass support in Herbal Energy
Seismitoad-EX
Super Scoop Up
Phantom Forces
Psychic support in Mystery Energy and Dimension Valley
Staples like Sycamore, Tierno, AZ, VS Seeker, Battle Compressor, Enhanced Hammer, and Xerosic
Metal support in Steel Shelter and Metal Links
Zubat line
Night March
Primal Clash
Support for vast amount of types
Wonder Energy and Shield Energy for Fairy and Metal respectively
Maxie and Archie for Fighting and Water respectively
Stadium support for Fire/Fighting and Water/Lightning in Scorched Earth and Rough Seas respectively
Introduction of Ancient Traits
Splashable Pokémon like AT Bunnelby, Solrock, and Milotic
Wailord-EX
Staples like Teammates, Birch, Rare Candy, Weakness Policy, Escape Rope, and Dive Ball
Roaring Skies
Support for Dragon and Colorless in Double Dragon Energy and Winona respectively
Staples like Trainer's Mail, Steven, Switch, Ultra Ball, and Wally
Mega Pokémon support in Mega Turbo
Shaymin-EX
Splashable Pokémon like Deoxys
Ancient Origins
Support for Darkness and Lightning in Dangerous Energy and Flash Energy respectively
Staples like Lysandre, Hex Maniac, Level Ball, and Forest of Giant Plants
Vespiquen
Eeveelution
Giratina/Vileplume
Volcarona
Hoopa-EX
Anti-EX support in Regis
Splashable Pokémon like Unown, Lugia-EX, and Relicanth.
BREAKthrough
Support for Fire in Burning Energy
Staples like Brigette, Assault Vest, Float Stone, Fisherman, Heavy Ball, Judge, Skyla, Parallel City, Professor's Letter, Super Rod, and Town Map
Introduction to BREAK Pokémon
Splashable Pokémon like Zoroark, Mr. Mime, Smeargle, and Octillery
BREAKpoint
Support for Water in Splash Energy
Staples in Misty, Sycamore, Fighting Fury Belt, Max Elixir, Max Potion, Tierno, Reverse Valley, Delinquent, Bursting Balloon, and Psychic's Third Eye
Splashable Pokémon like Garbodor and Hypno