There are lots of pierce and slash resistant monsters in IWD so you'll want to keep that in mind when choosing proficiencies. There are many good flails so I'd probably put flails/morningstars at the top of the list. I agree that there are a ton of good longswords as well so that's one you'll want to cover as well.
Comments
Flails/Morningstars are also quite good. It's possible to get a +2 flail with +1 attack per round as early as Chapter 2 if you're lucky.