There's been a million "best class for each character" threads, so let's try doing things from a different perspective. Who makes Holy Knight the least bad? Which Wyvern Lord is the most OP? I'm mostly looking at Master classes here, but a few Advanced ones that people tend to use or that don't really have a "better" Master version would be nice as well (Sniper, Paladin, Assassin, Dark Bishop, maybe Hero for the hell of it? Fortress Knight?).
Throw in DLC classes if you like - they kind of cheat by having characters tailor-made for those classes, but you could try War Cleric as well as War Monk.
"Byleth" is the cheat answer, so let's avoid that. If you pick a route-exclusive character, maybe include a runner-up that's recruitable.
To start us off, Lysithea being the best possible Gremory is pretty uncontroversial, as far as I can tell. Most people seem to agree that Hubert (with Linhardt as recruitable runner-up) is the best Dark Bishop, as well.