Crossbow, most bows at headshot, lance and I think you can onehit less armoured people in warband with glaive, but I've rarely used two-handed weapons while on horseback in warband, in fact rarely played khergits since the competitive community just never used them, so I can't be sure.
Everyone with slightest amount of brain try not deal in absolutes. What's the point of taking a 2h weapon if you can't even onehit anyone? You sacrifice your protection against projectiles and huge protection the shield provides in group melee for only one thing - the damage. No one will bother with glaves or menavlions if they won't be able to oneshot.