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FRLG is a good starting point since it's pretty linear in map design and simple enough in setting and story for anyone to get into. The only problem is you HAVE to hope the person in question talks to every NPC because I can see them getting stuck on the Tea sidequest or the Silph Scope requirement for the Pokemon Tower.
Emerald is good due to it having a someone cooler presentation than FRLG plus a cool setting and soundtrack to lull them in with. The game encourages a little bit more exploration skills than FRLG but also rewards the player with rare Pokemon, cooler Legendaries, and hidden items.
Also the gave gives you lot more stuff to do besides the main campaign like Contests, Berry growing, Legendary sidequests, and the Bottle Frontier - obviously.
I would also but X&Y and SwSh on the list but that's only if the player if fine with a mid story.
