I like Fire Emblem Engage. It's the culmination of so many great things in one game.
Vander is incredibly dapper and also one of the best, if not the best, Jagens in the entire franchise. He has good accuracy and decent strength so he can chip enemies down but not enough speed to double nor enough strength to accidentally kill. He has enough HP to take hits for weaker characters and can bait enemies toward him for your weaker units. His class skill/passive is unfortunate but it's the only thing subtracting from his perfection. He does exactly what you want a Jagen to do without being useless or overpowered (yeah, Seth and Marcus, I'm calling you out).
Every unit outside of Vander is viable.* Not only that, but you're getting a constant stream of immediately useful units. Whether you're playing an Iron Man game where you don't reset on unit death or you've just had bad RNG for random growth, the game helps you by giving you new, viable units the entire way. Some units require more investment than others but it's nowhere near previous installments where you could dump 30+ levels into a character and they'd still end up bad.
The map design is amazing. Chapter 4 might be one of the best-designed maps in franchise history. You have two units holding out against enemies that you have to get to. You're encouraged to be proactive and split your army right away. There are optional objectives that can be risky to go for. There's terrain to work with or around as well as choke points to use or defend. The enemy even gets reinforcements that throw you for a loop. It's such a great map and Engage has lots of great maps.
The game is colorful and bright. Each character has an interesting, unique design that makes them distinctive and they're placed against a constantly changing backdrop of green, blue, red, black, yellow, etc. There's a lot of variety in what you're looking at which not all past Fire Emblem installments have done quite as well.
The time rewind is a great mechanic. Nothing is more salt-inducing than playing a very long Radiant Dawn or Fates map only to have a mistake or unlucky crit at the last minute cause you to lose a unit forcing you to restart the entire map. The ability to negate bad RNG on long maps lets you enjoy the map sans frustration.
SP is super nice. Fates asked the player to do some very complicated cast pairing and child-planning to get "the good abilities." Three Houses forced you into hours upon hours of classes and monastery management. Being able to equip any ring, gain SP, and buy whatever you want for your units is very refreshing. You spend less of your time doing things that almost feel like chores and more of it just playing the game.
Emblems fix things. If I need a unit to be faster, I can staple Lyn to them and they are faster. If I need a unit to be tough, I can duct tape Ike to their back and they are tougher. It's an unparalleled level of character customization and between buyable skills, stat boosts, and engage skills it opens up so many great options.
Maddening is good. The Fire Emblem franchise has a lot of ups and downs with difficulty modes. Some games get it right, some games do not, and some get it very, very wrong. Maddening in Engage does it right and fixed growth is excellent as it lets everyone share the same experience to an extent. You no longer live and die by the RNG.
There are more good things to say and some bad things too. Some of the story complaints are a bit overwrought. As someone who is (at the time of writing this) replaying Fire Emblem Fates Revelations, there is an entire universe of bad writing that Engage isn't anywhere near. Anyone 0/10'ing Engage's story is just being dramatic.
* We're going to quietly pretend Jade/Maddening doesn't exist and just about every unit on the next 2 maps after joining doesn't double her.