User blog comment:Fieryno/I'm new - Unit of choice/@comment-26947994-20180407232757/@comment-26947994-20180408234005

Shion Lead -  cause he's a good leader

Zegstia - Lots of nice attacking buffs, will increase your damage by a lot

Phileine - For heals and some BB gauge momentum

Stein - For the mitigation

Segrud - For defensive buffs other than mitigation

is what I'd recommend. If you don't know, "mitigation" is the buff that cuts all damage in half. It's very overpowered, and if you don't have it, you basically can't take on any challenging content.