Depending on the experience you’re looking for I’d have different advice. If you’re looking to play efficiently, you should make good use of Titania. You get her in the first real chapter, and she has amazing bases coupled with good growth rates. She’s sort of like a mini-Seth. Oscar, Boyd, Kieran, and Jill are some of the best units that you’ll want to focus on training because they all become very powerful in different ways but most importantly can all use axes post-promotion, which are the best weapon type. Mounted units are also really good in general because they get fe5 canto, but they’re not so broken that unmounted units become irrelevant.
On the other hand, if you’re just looking to play the game more casually, really just do whatever you want. The game is easy enough to beat with just about any team (I solo’d the last few chapter with Mia, the worst unit, in my first playthrough) and you get bonus exp to feed to any underleveled units you want to train.