The most powerful warrior.

Archers are good, of course, but in a one-on-one fight, and without a hero's influence, they are weaker than Minotaurs. Minotaurs kill them before they can kill the Minotaurs (verified in online play). Minotaurs have better defense and more hit points, and they can move quite far, reaching the archers in 2 turns. Archers only manage to fire twice, and they are weak in close combat, so they lose. The situation is practically the same with Swordsmen; due to their hit points and defense, they win (although it takes them 3 turns to reach the archers, which has also been verified). The same applies to Bears; they win due to their hit points, speed, and defense (and this has also been verified). So, we can draw one logical conclusion: in a one-on-one fight, archers are weaker than all three. However, if they are part of a hero's army of their own faction, they have very great potential and strength, especially if someone is protecting them.
As for upgrades, I haven't checked, but I think the same applies.