Who's in your party?
Flails.
No NPC comes with flail proficiencies, and one of the strongest weapons
in the game is a flail.
Also if you are good aligned, longswords are a good choice for your PC if
you plan to keep a certain Rashemani ranger / hamster duo in your group;
otherwise 2-handed swords is a good choice. But flails is a must.
If you are evil you can take the 2-handed sword proficiency yourself, though
there is nothing wrong with longswords. But flails is a must.
If you want a spoiler, read my thread on How to kill a lich? for a
kick-ass longsword that (using some of the methods suggested in there)
you can get quite early in the game.