I find that Heretics are good for the siege due to thier armor, shield, and good weapon.
For cavalry you might try Shahir Lancers. A charge with their heavy polearms can be vicious vs any type of troop. Crusaders are close to that level as far as punch, + they have more sustained endurance in skirmish. Thing is though that the lancers can be massed as they are more easily achievable on account of their lower spot in the tree.
Heretics have to be the best in regards to being bogged down in melee. They will be the cavalry of choice vs heavy infantry in rough terrain. Sultan's Bodyguard are a close second in this category.