I would say Gen 4, but mainly DP, and perhaps Platinum.
The physical/special split was finally introduced, and this actually added viability to a tonne of Pokemon who were previously crippled by their own typing and moveset; Gyarados comes to mind. All of a sudden it gets to make full use of Waterfall, Crunch, the newly-introduced Ice Fang, where previously all of those moves would've drawn from its lesser Special Attack.
Additionally, it's just so damn slow. Game Freak's optimisation issues didn't start in the Switch games; surfing from Sandgem Town to Pal Park, or Sunyshore to Victory Road, should not take nearly as long as it does in DP. Battles are also painfully slow, even with animations off - there are obnoxious pauses when a move is selected, and the HP bar takes forever to move.
It's also possible to clip through the ground in Jubilife if you cycle too fast.
So, Gen 4 (DP) is the hardest to play/get used to.