It's a tie between Alola and Sinnoh. The games were awesome, as were the Pokémon. Plus, the games led me to both my favorite Pokémon (Popplio) and my favorite middle evolution starter (Grotle) of all time. In fact, if I ever complete a Pokémon game in Sinnoh, I plan to evolve my Turtwig into a Grotle… and keep it there.
I never thought of it before, but mine has to be Unova. Each area feels like it brings something new to the region, and it's got some of my favourite routes.
Hoenn or Kalos. Generation 3 was when they first introduced contests. Plus my favourite starter Pokémon, Torchic, is introduced. I like Kalos' design, and Mega Evolution was introduced, and some of my favourite Pokémon(Blaziken and Pidgeot) receive Megas.
I love Sinnoh!! Honestly, I love all the cities and towns, with Snowpoint being my absolute favorite. I love all the Pokémon and the beautiful resort area post-game! Absolutely stunning geography.
Hoenn is my favorite. It contains many of the Pokémon that are my favorites and I love the overall esthetic. I played gold but felt that the small pixels didn't do the world justice. However, I feel like emerald did the Pokémon world justice giving a distinct and beautiful Pokémon world with unique mechanics. I would love to see diving on the ocean floor return someday.