Favorite regional concept

Oh boy this is a real toughie. It boils down to two Generations for me: Gen 4 and Gen 6. Gen 4 brought the Physical/Special split, and I cannot tell you how immensely impactful that change was for not just the new Pokémon introduced in Gen 4 but those from Kanto, Johto and Hoenn respectively. It made Pokémon like Flareon actually "viable" since its 130 Attack could finally benefit from STAB. Similarly, with Gyarados(post Gen 1) where Waterfall tapped into its 125 Attack or even Azumarill and its Huge Power ability being able to tap into its Attack Stat with Physical Water moves.

Gen 6, on the other hand, introduced Fairy Types, changed old Pokémon into Fairy Types(making Clefable a true monster) and added Mega Evolution. Mega Evolution instilled new life into old Pokémon and none more than Mawile. Not only did it gain Fairy Typing to go with Steel(best combo bar none), but its Mega Evolution also gave it Huge Power that took it's respectable 105 Attack Stat and effectively made it 210. Meaning it hit like a ton of bricks when it moved. That is why, in answering this question, I'm going to roll with Gen 6 and Mega Evolution.

Z-Moves. It was a simple feature but it gave all pokemon a dynamic scene and the special ones were awesome.