5th Gen Black 2/White 2

-Best Story
-Best Gameplay
-Best Pokemon (unpopular opinion)

B2W2 is in my opinion the perfect Pokemon experience.

RSE were amazing. The music, the awesome new Pokemon designs, it was all just so good.

2nd gen, Pokemon Gold, Silver, and Crystal.

Silver. Man that was awesome. When I was able to go to Kanto I was like, WHAAAT?! Totally awesome game. I should really pick up Heart Gold or Soul Silver.

Heart gold is my favourite. 2nd gen is what I'd say with crystal but sicne HG is gen 4 even if it is a remake its still gen 4.

It's tough, I like all the gens. I would go with II because I had the most success there. Plus, going back to Kanto and facing Red, who you once were, was chilling and exciting.

Gen I was a great start and my childhood, but realistically, it's broken in some ways (damn those Psychics).
Gen III and VI were solid experiences that provided new worlds and new gameplay experiences to the established gameplay.
Gen V and VI were great as well. Creating new energy for me as an older and more experienced gamer. The new Pokemon, the Megas, the graphical evolutions. You can just tell that V was sorta teasing how mainline Pkmn games could potentially be when the environments get out of their 2D overworlds.

2nd gen, in particular Soul Silver/Heart Gold.

Gen 2 easy. It was the best gen with the most new additions and extras. None of the following gens have been able to reach the standards it had set. Gen 3 was the restart moment, where it stripped a lot of things that made Gen 2 great, without leaving much else new to enjoy. : P

Gen 2, followed by Gen 6

Gen 1 because it was fresh.
Gen 2 because it perfected everything before the series got stale.