Bastard sword's my favourite, no question.
It's like a medieval swiss-army knife: it's got reach, speed, you can wield it with a shield on horseback and against crossbowmen/sea raider axe chuckers, and when things get nasty you can ditch your shield, get a good grip with both hands and chop khergit horses in half.
Although I've recently become a fan of the humble spear: its reach is pretty woeful (seems to be 'just' ahead of the horse when couched) but it's well fast for a polearm, you can ride just behind and to the right of a pesky horse-archer so he can't shoot you and just poke his horse to death. And it looks awesome with the top three feet covered in blood.
But the best weapon I've found? My heavy hunter, called 'Roadkill'. When I'm facing hordes of river pirates I sometimes don't bother swinging a weapon at all..