It takes a very strong swing to cut their head off...and it's not like one on one...or ooooo knife and butter I can do it all day.
I wouldn't go for a decapitating blow, I'd go for the smash, hence the broad sword or mace combo. It doesn't take an extreme amount of force to smash the skull and brain inside.
If was going for dismemberment I'd choose a Japanese katana.
Actually I do. I have quite a few swords and semi-formal training with them. Ya, I'm a mid-evil warfare nerd.