So, it honestly depends on what you're looking for in a Fire Emblem game.
Do you like giant maps where cavalry are king, mages are OP and the story is great? Then play Genealogy of the Holy War (FE4). Do you like smaller maps and don't want to have as many outright useless units or to have to fiddle with eugenics, but otherwise want the qualities listed above? Play Blazing Sword (FE7). Do you like branching promotions and the ability to grind indefinitely, or think those other games are a bit too hard? Play Sacred Stones (FE8). Do you want to solo the entire game with your lord character? Path of Radiance (FE9) is for you. Do you want to see the only example of a mage lord we had before the Avatar system? FE10. Do you main Marth in Smash Bros? FE3. Do you main Roy and want to play a worse version of FE7 in almost every way? FE6. Do you hate yourself? Play Fire Emblem Fates and see the mangled skeleton of what could have been a much better game.
I haven't played Echoes or Three Houses yet so I can't judge them. I also haven't played FE5 all the way through, but I hear it's good if you like high difficulty and the ability to have anybody steal weapons.
Do you like giant maps where cavalry are king, mages are OP and the story is great? Then play Genealogy of the Holy War (FE4). Do you like smaller maps and don't want to have as many outright useless units or to have to fiddle with eugenics, but otherwise want the qualities listed above? Play Blazing Sword (FE7). Do you like branching promotions and the ability to grind indefinitely, or think those other games are a bit too hard? Play Sacred Stones (FE8). Do you want to solo the entire game with your lord character? Path of Radiance (FE9) is for you. Do you want to see the only example of a mage lord we had before the Avatar system? FE10. Do you main Marth in Smash Bros? FE3. Do you main Roy and want to play a worse version of FE7 in almost every way? FE6. Do you hate yourself? Play Fire Emblem Fates and see the mangled skeleton of what could have been a much better game.
I haven't played Echoes or Three Houses yet so I can't judge them. I also haven't played FE5 all the way through, but I hear it's good if you like high difficulty and the ability to have anybody steal weapons.