How good a game is is relative to its time. Obviously 3rd and 4th gen are far improved when it comes to many gameplay mechanics, but neither of them feels like a quantum leap. The biggest improvement game from 4th gen's Physical/Special split.
When you look at 1st gen, it was the pioneer, and it was pretty much a revolution in gaming. It was pure genius, and when it took our world by storm, Pokémon was basically the religion of every child for about 2 years. Usually games never live up to the prequel, especially if there's no real jump in technology. This is where Pokémon GSC prevailed. 2nd gen was such a giant leap above 1st gen in every way, that it just felt like the most incredible and complete sequel you could have ever hoped for. More Pokémon, new types that balanced Psychics, a new land, returning characters/Team Rocket win, returning to Kanto, trading with old Pokémon games.
All of this was fantastic, and then came 3rd gen. Although it was due to the upgrade in technology, the lack of backwards compatibility sucked. There were almost no old Pokémon you could catch in RSE, and sequels always provide a decent amount of fan service by at least keeping your old favourites in. 3rd gen Pokémon were for the most part, uninspired, a lot of them didn't really "feel" like Pokémon. Maybe it was the shift in design style, who knows. However, 3rd gen brought some fantastic thngs like natures and double battles, adding more depth to the gameplay. If you ask me, Pokémon Colosseum was one of the most well designed and thought out Pokémon games. Orre was pretty bad***, Wes was pretty bad***, the bad guys were pretty mean. Orre was a much more realistic world than any other, apart from the names (Eagun, Beluh, Dakim, Nascour... yeah),
4th gen was was hands down just better than 3rd gen. It introduced the physical/special split for moves, which was the best overhaul anyone could have possibly thought up. The Pokémon were pretty nice this time round, they added evolutions for rather patetic UU Pokémon, giving them a use in competitive play. They made it easy to perform quick EV training, they made a much better overworld. The only gripe with 4th gen is Stealth Rock, which is probably the most broken entry hazard you could conceive.
Neither 3rd or 4th gen were quantum leaps though, neither of them felt like major advances in the series. Neither feel quite as incredible for their time as 2nd gen. This is why 2nd gen is the absolute king of Pokémon generations, and will probably remain so unless Game Freak pulls a 100lb rabbit out of their hat.
tl;dr: I have no clue how to summarize this. Read it you lazy chump.
