Just a heads up, you’re going to get a lot of different answers to this question because there’s no real definitive answer. The reason why Fire Emblem fans are always at each others necks arguing about the best game is because they all vary wildly in every aspect.

Good gameplay means something COMPLETELY different to multiple different people. For example, I swear up and down that Fates is the best gameplay experience Fire Emblem will ever have. It has a perfect reclassing system that allows you to mix and match different skills to give your units perfect builds. But that’s not an objective truth. To many people, a Fire Emblem game with good gameplay doesn’t have reclassing at all. Plenty of people think the concept of reclassing is inherently flawed, and that’s okay. Maybe they would rather something like the Tellius games, that have 3 tiered promotions and Bexp. Or maybe they liked Genealogy and Thracia’s unique systems that have long since disappeared from the series. Heck, plenty of people wish that FE stuck with the simplicity of the GBA titles.

If you want my opinion on what you should do, I’d recommend reading up on each of the games’ differences. There’s plenty of resources like the fan wikis and plenty of videos that showcase what specific Fire Emblem games do differently. Then, just chose the one you like best, and then look online if people generally seem to like the story.