Personally I like the Lut Gholein Spearman best, since its defence is the highest, and u can equip him with strong spears, he'll attack at lightning speed with his Jab ability anyway, and he can use strong armour like the Barbarian Merc, plus he helps a lot with the magic auras, which is something that nobody, not even the barbarian merc, can equal. I find that this is the merc to use whatever class u are.