Generation 2, by far. I don't know why it gets so much praise. It easily has the most bland starters in the entire series. Meganium is just an inferior Venusaur. Typhlosion is boring compared to all the other Fire-starters, and Feraligatr looks like a plain crocodilian where they just tacked on some spikes and called it a day. While it did introduce some good mechanics and features, it barely featured the new Pokémon (just look at the Pokémon the gym leaders use), and many of them weren't too great as far as usefulness goes, with Unown arguably being the most useless Pokémon of all time in battle. The level scaling was pretty bad as well. Johto was pretty empty for the most part. You can tell the inclusion of Kanto costed them. I just just consider it an extension of Gen 1.
At least Ruby/Sapphire felt like a real sequel with more features that were important (Double Battles, a refined EV/IV system, Natures, Abilities etc), yet you don't see as many people praise Gen 3.