Polearm or Zweihander. Bullcharge all the things and hope I'm not dead before I get the chance to swing, because if I can swing before they can, they're so dead.
One of the most effective weapon combinations I've seen is a shield, strapped to your left arm rather than a buckler style shield... Use your free left hand to guide the polearm (spear or glaive) and your right to thrust and row (think rowing like a boat... You get the leverage, and no matter what you hit your opponent with, it's going to hurt... )
Aside from that, these troops usually stood in a line, behind swordsmen with shields, but in a one on one battle, a skilled spear man could (like the spartans) pretty much annihilate anything that stood in their way...
With this particular setup, you would include bracers, or vambraces, as well as thick shin guards, and a single pauldron on your right shoulder... (if you put one on your left, it blocks the shields movement, and could possibly low you down)
Archer: Charge your opponent, taking time to watch where the arrow is aimed, and blocking with the shield accordingly. Upon reaching striking distance, simply close in and eliminate him...
Spear- Man: A Spear - Spear fight requires care, and strength to manipulate your opponents spear so that you can slide the tip of your weapon into his guard and harm one of his hands or arms... Once you manage to take out his forearm, you can finish him off easily...
Knight: Take GREAT care, and circle your opponent, keeping the full length of your spear between you and your opponent. You are lighter, and faster, and must use your speed, against his armored immobility... Slowly tire your opponent, and strike at his legs with the full length of the polearm... If you manage to strike his leg and do harm, you can simply bash his shield or sword until they break with the overwhelming power of the leverage your polearm offers you.
Swordsman: The greatest challenge for a spearman... His mobility allows him to quickly overtake your spear / glaive and shield... If you do not maintain a distance from him, he will slide under your guard and quickly dispatch you. The best defense against this, is to wield at half shaft, keeping your weapon aimed at him, threatening impalement if he charges... You're holding at half shaft, meaning he cannot sweep the tip aside with his blade, nor with his shield... His agility means that he can match your steps though, and if you do not keep your distance, he will close in...
Attempt to maim a leg, or if possible, a foot, stealing away his mobility. If you cannot, use the spear or glaive to manipulate his shield... The strength of your polearms strikes may break his arm, or the shield itself (Straps)... Once one of these goals is finished, he will either charge without his shield, which is suicide, or have to take a knee from his injured leg... Also leading to death...