There is no best team, it depends on your playstyle. My favorite team is CAAA 
Archer's advantages: (a) GM in Bows (E in Bows at the beginning of the game). Archers can hit your opponent seriously sooner than the opponent can do something to you. (b) M in Spears. Spears are the best weapons in MM7. (c) GM in Perception. No problems with hidden doors. (d) M in elemental magic. Archres can emulate sorcerers in the end.
Archer's disadvantages: Rather difficult first promotion. Okay, a single Scroll of Invisibility can do the trick - if you know exactly what to do.
Thief's advantages: (a) GM in Disarm Traps (E in Disarm Traps at the beginning of the game). A Thief can reduce (and, in the end, eliminate) chest blasts without need of super strong supporting magic. (b) GM in Daggers. This makes a Thief equal to a Knight as far as the inflicted damage is considered.
Thief's disadvantages: Difficult first promotion. Nothing helps here. You must fight a battle against "cans".Note that you must open chests somehow. Archers are very inefficient in it, even with strong supporting magic. Well, in the end, the chests will stop blasting. A Thief is the best solution here but, IMO, thief's overall value is considerable lower than the archer's one.
Choose yourself.
