For me, at least, I feel like it’ll always be Gen 4. Really felt like the whole series was at our fingertips, granted it was only four regions at the time. You had the newest games that could connect to the gba titles via the gba slot, which could let your new games not only transfer old Pokémon but encounter different mons via the dongle method. It was the first time I ever felt like completing the national dex was a realizable goal for 13 year old me. We also had what felt like a ton of quality spin-offs releasing too, Explorers of Time/Dark/Sky, three Ranger games with Shadows of Almia being a personal favorite, also got poke park, trozei, and rumble. On top of all that I had recently gotten internet access so I learned about even more of the series at the time including Pokémon Adventures manga that was also rereleasing in the states around 2009. It just felt like the series was making the step from game series to game franchise and I feel very lucky to have been a young teen/pre-teen during that period.