Gen 4 is the best gen because it's the only one that doesn't rely on Gen 1 and, as such, as a unique region as a result.

Gen 2 literally only introduces 100 new Pokemon and has Kanto as a region, with the previous MC and rival as main bosses.
Gen 3 is more unique but half of the dex still relies on Gen 1 Pokemon and a Mew event.
Gen 5 is literally a Gen 1 remake. The Dex is a 1-to-1 copy. It's literally trying to emulate the Gen 1 experience instead of being unique on its own.
Gen 6 has Viridian forest, gives you a Gen 1 starter, Mewtwo as the final boss and most of the megas went to Gen 1.
Gen 7 has Alola forms for Gen 1 mons, Giovanni as the leader of RR, Kanto gets name-dropped constantly.