The Pokémon from those gens are already part of the reason why I consider Johto and Sinnoh my joint-favourite regions, but I might give Johto the upper hand here, Gen 2 has many of my favourite Pokémon and very few ones that I don't like, and overall my favourite starter trio design-wise (although not my favourite individual three starter( familie)s, even if the Chikorita family is one of those). Some of my favourites include the entire Chikorita and Cyndaquil families, Croconaw, Furret, the Ledyba family, Lanturn, Mareep, Ampharos, Azumarill, Hoppip, Jumpluff, Umbreon, Heracross, Teddiursa, Corsola, Mantine, Skarmory, the Houndour family, Smoochum, Tyranitar, and the Legendary Beasts are my favourite legendary trio - and I particularly like Suicune.
I can mostly say the same thing about Gen 4 but it has too many legendaries and I have less Gen 4 favourites than Gen 2 ones.
As for the other generations, Gen 3 probably comes next, even if it may be because by introducing more new Pokémon there are also more that I find less interesting. Gen 6, 5, and 1 have a mix of iconic designs and others that I don't like much, Gen 7 isn't bad but I'm not really a fan of the Ultra Beasts and so far Gen 8 is the one I like the least, though it may be because they're still so new.