In terms of the main faction troop tree for infantry troops, the best heavy armoured infantry are the Khuzait Darkhan, Vlandian Sergeant, Sturgian Veteran Warrior, and the Imperial Legionary. All four have 130 skill level in one-handed, polearm, and athletics stats, and perform similarly on the battlefield. The Vlandian Sergeant may have the edge however, as they tend to spawn with heavy maces in their loadout. These provide them with significant blunt damage, which is extremely effective against other heavily armoured infantry. The Khuzait Darkhan have the most variety in terms of weapon use, with a polearm to counter cavalry, a powerful sabre which can cut through multiple non-heavy armoured troops swiftly, and throwing javelins. In addition, they have a shield which they use with the sabre, making them effective both in large field battles and in sieges, where shield defence is vital for an attacking force.
If you’re fighting an open battle where defences aren’t so important as they are in sieges, the Sturgian Shock Troop is one of the best options for heavy infantry. They aren’t among the best heavily armoured troops by default, with slightly weaker defences and less versatile weapon options than the Khuzait Darkhan, Vlandian Sergeant, Sturgian Veteran Warrior and Imperial Legionary, but they make up for this with superior skill in one-handed and polearm combat. In particular, their use of the Warrazor polearm makes them extremely effective when it comes to cutting through exposed enemy lines, and they will almost always win out in this scenario, even when outnumbered.
In terms of the best two-handed, versatile damage dealers, the Vlandian Voulgier and the Sturgian Ulfhednar are perhaps the best options. The Vlandian Voulgier is so powerful because they each come with a polearm to counter cavalry charges, but can also switch to a powerful two-handed weapon for close quarters combat. In addition, they have throwing axes which they can use to their advantage by damaging incoming units before they clash in melee. On the other hand, the Sturgian Ulfhednar has superior base damage output, athletics, and throwing skill, with 150 two-handed and athletics skill level compared with the Voulgier 130, and 130 throwing compared with 80 for the Voulgier. This makes them much more versatile on the battlefield, but they do suffer for it with significantly weaker defences.