I only played Conquest and Revelation. Skipped Birthright because I was already a veteran of tactical RPGs. Honestly, Conquest is one of my most favorite games of all time. I'd be beating a dead horse for mentioning the replayability and depth pointed out by everyone, so I'll also mention that I found the characters likable in their designs, demeanor, and quirks. YMMV, but hey, preferences are preferences.

Here's what bewilders me about the fans who cry foul on Fates' writing. Why the hell would the wrting/cutscenes make or break your experience of playing Fates? I'm serious, whatever happens in the cutscenes has little or nothing to do with what the player actually does. The game doesn't let you explore Nohr or Hoshido, the game doesn't let you interact with anyone outside of your army as NPCs.

Think about it, you live in an interdimensional castle town where you caring for it and the army who becomes its citizens. Including your spouse and child. And in Conquest, you had to be decisive, resourceful, and determined to see each battle through to the end. That doesn't sound like something a Mary Sue has to do, that's something a tactical badass would do. Something that the player experiences for themselves instead of a script being thrown into their faces. Support conversations are fine though, since they're even more optional but give a bit of flavor to characters I like as a reward for using them. Some worth a cheap laugh, others making me appreciate my favorites even more.

Also, I never understood why Corrin gets hated a lot when really I just found them typical. But because of player agency/freedom, they can be molded into a more interesting persona than whatever the writers dictate. Player headcannon is going to have a more explicit impact on my experience playing the game than writer cannon. Even outside of that, Corrin's outfits look cool and they're fun to actually play as.