2h get the biggest bonus for creating stun(1.4), polearm is a bit below(1.3) and 1h is last(1). Interesting enough blunt gets another multiplicative bonus of 1.5 while the other damage types have a 1.
In the end there are only 2 types of stun: no stun(0.02s) and stun(0.52s). Which of both takes place is decided with a rather big comparision of attacker and defender weapon/shield with a lot modifiers like swing direction, item weights, hold bonus, etc. including the above mentioned stuff. Also there is no stun on quick swings as it seems. You have to hold your attack a bit to cause stun, no matter if it is a maul against a stick.