[SixOfSpades]

All classes start with a THAC0 of 20. In order of best to worst:

Warriors lose 1 THAC0 point per level, and gain a weapon point on levels that are multiples of 3.
Priests lose 2 THAC0 points every 3 levels, and gain a weapon point on levels that are multiples of 4.
Rogues lose 1 THAC0 point every other level, and gain a weapon point on levels that are multiples of 4.
Wizards lose 1 THAC0 point every 3 levels, and gain a weapon point on levels that are multiples of 6.

Priests and Rogues have essentially the same THAC0 breakdown. Also remember that a Rogue will level up faster than a Priest, so they're actually better fighters, except for the armor issue.