Depends on what you want, have not played far enough myself yet for solid advise. If you go for a the standard route of leveling up and ending them in a high tier, consider leveling them up all the way, as this game has very low growths. However, as I understand it, some of them can become a Dread Fighter, whom you can propel into a promotion chain loop, first back to Villager for another round of leveling towards Dread Fighter, and so on.
Possible Dreadfighters on the Alm route; Kliff, Tobin, Gray
Possible Dreadfighters on the Celica route; Atlas, Deen, Jesse Kamui, Saber