Pokémon Emerald. It was the first one I played and still go back to playing it. Gen 3 overall is my favorite, really. I still have the green cartridge (though no gameboy to play it on).
Pokemon Diamond and Pearl. Probably because that was my first. The funny thing is that I didn't even realize that there were games and I thought that it was just a TV show and when I saw Diamond on the store shelf for the first time I thought to myself, "Wait, they made Pokémon into a game?! I need it!!!" Replayed it last year and it brought back a lot of old memories.
Heart Gold and Soul Silver are probably the best. I cannot recall a game with two regions. Platinum and Diamond hold a special place in my heart too though.
I only played pokémon black and white and a little bit of pokémon go
I don't really have an opinion on pokémon related subjects anymore but if I had to choose it would be the black and white series
I'd say Emerald, so I thoroughly enjoyed ORAS, but gen 5 holds my favorite poke, so I'm pretty torn. S/M; US/UM are fun, but not as much as ORAS, imo.
Aaaaaa, but Sinnoh!
Dammit, I like all of them.
Emerald was my favorite for sure, I'll never forget climbing that tower for Rayquaza in some random restaurant when I was like 6. I think I spent the most time in Diamond though