Technically, everyone, the strongest character in the game is Magnus himself, but you have to WANT to do this.
Magnus, when he starts chapter 4 as a General, gains MUCH better stat growths then his previous 2 classes, so if you leave him low level for the entire game and then over-abuse Training on him, he'll be pretty much a complete killing machine.
My friend actually did this, Magnus, in the Calibur mission, was able kill both of Destin's dragons and ALMOST finished Destin himself, SOLO, nobody else in his unit.
But yes, as a general rule of thumb, Ankiseth and Destin are probably the strongest units in the game.