>>20584010>>20584162I'd go with fifth gen. Every other gen added something new and relevant to the series.
Gen II added breeding and other important mechanics
Gen III added more important mechanics such as double battles and the graphics got a huge overhaul.
Gen IV Improved the graphics again. Added internet connectivity and improved on the battle system by making the physical/special moves split.
Gen V Added two new gimmicky battle modes that are barely used at all except for the novelty of it. Didn't really add any other mechanics and graphics got worse.
Gen 6 Had another big graphical update. 3d battles, character customization, Mega evolutions etc.