Between 4th gen and 5th gen, which should I play?
I played all the other gens in this order. Red, Ruby, X, Crystal, Yellow. Which would be better to go into next?

For whatever it's worth, coming back to 2nd gen was weird. Mechanically, it was a lot better than I expected it to be. However it felt too much like first gen. Nearly all the NPCs exclusively had first gen mons and many of the areas had maybe a single second gen mon that appeared at a very low rate.