If I should talk about heroes that can be good standalone units in many different teams then I would say Camus and Kagero. Xander is an honorable mention but I feel that he shines mostly thanks to cavalry buffs.
Camus - he's a great frontliner even outside of cavalry teams, mainly thanks to his good defense and speed. Vantage makes him into a big problem, that can't be dealt with sometimes. Also he's fun to play, because he moves fast and hits hard, which enables many different approaches to team building and in-game tactics.
Kagero - the ultimate antimage, and with +spd she can run desperation with confidence. She's also fun to play thanks to huge bursts of dmg to infantry units and she has a pair of insane anime boobs.
About Nowi - she's good, that IV is not worth 20k and I find her boring so I will probably never upgrade my ones. Can't say much about her for that reason.