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Fire Red/leaf green -> Heart gold/soul silver -> Black/white -> Black2/white2 -> XY
Fire red/leaf green will give you a great primer as to what to expect from the games, and if you are excited about it, you can then play Heart Gold/soul silver which shows the evolution to expect from the next generations; additionally they are remakes with all the fixes and changes that were made since then. I feel like you can just jump to Black/white gives you a great new in depth story and black 2/white 2 can continue on with the evolution of the characters, teams, and region, then play XY so you'll know what to expect from SunMoon when it comes to the 3d transition.
Like many said, games from the original red blue yellow to black 2 and white 2 can be emulated but with the road map I wrote, playing legitimately will allow you to transfer your teams to the next game and into the modern generation of Pokémon games. I feel that this order will keep your excitement high for much longer; you don't necessarily have to play all these games to enjoy the next Pokémon game but rather if you loved fire red/leaf green as it gives you what to expect from a pokemon game and want more, then go to heart gold/soul silver, and continue in that progression via the road map.
Hope you have fun!
