Gen IV for me. Gen III was my first with Emerald, and that's probably my favourite individual game. I bought LeafGreen as well and its a good game - very streamlined, refreshing given the heavy focus on legendaries from Hoenn onwards - but Gen IV was my peak Pokémon. GTS and Wi-Fi trading opened up the possibility of finally evolving a Kadabra (in my earliest saves when I didn't understand that Pokémon evolved any way other than levels, I got one to lv. 100), and completing the Pokédex (I was actually pretty close to getting all 493 as I had the events, but had only one DS so needed to trade through a user on here...he never gave them back). I didn't enjoy the updated Contests in Gen IV, or how slow Diamond / Pearl were, but in every other manner I preferred the gameplay (especially the Physical / Special split).
HeartGold / SoulSilver are possibly the most Pokémon you can get in one cartridge: two full regions, about 300 Pokémon available in-game, two Champions (one the strongest NPC in Pokémon), a very cool event with Celebi, a fun Safari Zone (Hoenn's had the bikes which was kinda cool, as were the trams but overall Hoenn and Sinnoh's Safari Zones both sucked), and a great accessory in the Pokéwalker (which feels purpose-built for my current quarantine lifestyle). I could go on, but I want to play more (and yes, I'm in Gen IV).