Anyway voted for the 3D era mostly because of Gen 6. It had a lot of good stuff. Triples were still allowed to exist. Chespin. Megas, easier breeding mechanics, easier EV training, easier IV checking, the PSS, PokeAmie. Gen 6 had a lot of good things going for it. Gen 7 was cool mostly because of the dismissal of HMs and the change on graphics for the overworld sprite (now a proper model). Gen 8 only had as a saving grace further improvements on some mechanics and that's about it.
From an aesthetic standpoint, the Game Boy era wins and it's not even close. I almost guarantee there's some nostalgia creeping in here, but I have always preferred my games to have simpler graphics and stylization so the basic overworld and sprites appeal to me much more. I would lump Gen 3 in with this too; 4 onward is where things started to take a turn for me personally though I can't figure out exactly why. All I know is the more detailed things got, the less stylized it became to the point where the more recent games feel more like "generic anime" in style rather than anything original. Does that make sense?
Ultimately, though, I cast my vote for Gens 3-5 because at the end of the day we're talking about a game here and mechanics wise everything got ironed out to perfection in this era. Adding abilities and the physical/special split were huge depth enhancers and everything that came after is more of a tweak (Fairy type) or gimmick (Megas, Z-moves, Dynamax).
The GBA era was great but I think I liked the GBC era a little more because Pokemon was still new back then so everything felt more original. My favorite Pokemon is from generation 1 so maybe I'm a little biased too.
I hated having to buy batteries for my GBC but that was a small inconvenience.
I'll go with the Generations III - V, for their amount of content and introducing things like Natures, Abilities, the physical/special split, and reusable TMs. Generations I and II are just unenjoyable today, and Generations VI - VIII look too simplified for my liking.
Nostalgia: G1 grew up with the cartoon and the cards didn't have the games. The one time use moves which persisted until Gen 5 makes them so annoying (I wanna use that move but that other mon could also benefit. i hate those limits)
Stories: Gen 5... N's story was interesting and tragic. Ghetsis abused a kid who was gifted. Seeing turned into a speechless sore-loser wreck in Black and White 2 was so satisfying
Playability: HM Slaves Bother me in every game so Ultra Sun and Sword playability are my favorite era. Customization is also a big aspect.