LogFAQs > #978697842

4 was the peak. Physical/special split was a huge improvement to the core mechanics and the games reused the best content of every preceding generation, so they were packed with stuff to do. The games that came before still had some gen 1 jank to clean up. The titles that came after had little to add in terms of mechanical improvements, focusing instead on gimmicks that would show up for a few games before getting phased out.