You can start anywhere to be honest. None of the games connect (Except heart gold/soul Silver and Black/White 2) and each game is made to be your first. However if you are really wondering what you want.
X/Y if you just want to play the latest pokemon game
HeartGold/Soul Silver if you want to play the best one
FireRed/Leaf Green if you want to start where the story starts
Red/Blue if you want to be ultra hardcore and start from the beginning.
Pokemon Go if you are ultra casual