I'm planning on making a weaponmaster to take far into the Undermountain. The only character I've taken to epic levels is a Monk, fists only. So - any suggestions on the BEST weapon of choice for a WM that's in it for the long haul? By that I mean, best selection of high end choices, or that weapon type that has one specific version somewhere that is just too good not to be used.

Theoretically, when you take into consideration that the Weap Master can only get a mid level AC and thus you would ideally want to finish off your opponent quickly, the best weapon for the job would be either the two handed sword or the greataxe.

However, there are some monsters in the game which have higher resistances to slashing damage...

Anyone building a Weaponmaster needs to think about the Devastating Critcal feat.

If using Dev Crit it is worth looking at the weapons with the best critical spread (kukris, rapiers, scimitars) and preferably keen. They all exist somewhere in Undermountain, usually at those lower levels you can't get at until your character ia already well advanced.

Longswords are available everywhere and many are quite powerful.

Two weapon fighting increases the possibility of a Dev Crit hit but will mean sacrificing AC.

Sorry. That last post of mine didn't read as I intended.

What I meant was that the best of the kukris, scimitars and rapiers are hard to get at for lower level characters. Good longswords are available almost everywhere.

A character specialising in a weapon with a wide critical spread would have to use relatively weak weapons with low AB until it could reach the better ones at the deeper levels. Probably around character levels 20-25.

I am currently building a wm, and I'm using Kukris, dual wielding. There is a +7, Kukri of the Master available, and it is accessible to lower lvl chars, providing you have the gold.
The reason for the choice has been discussed here, critical threat range, using Dev Crit. This is not a dex based fighter, but a solid tank, using the best of the best heavy armor. The char's current threat range is 10-20. He has yet to achieve even overwhelming critical, which is of course required for Dev Crit, but is still scoring criticals in the 80 point range, and has had good success against the creatures he has encountered that are immune to critical hits.
I would strongly suggest that even though yes, there are a lot of long swords available, and some are quite good, the best weapon of choice should have a higher threat range for criticals, even if you would choose to not use the Dev Crit feat.

Great Axe, there's just no substitute for the one you get as a reward at the end of the proving grounds. A properly build Weaponmaster can score critical hits of 400 damage with it.

What about Bastard swords? The reason I'm asking is that it seems you sacrifice a lot by using a two handed weapon. Instead of two items with additional effects you only have one. A bastard sword seems to get the most damage for a one hander, and with a predominately fighter character I wouldn't think the extra feat is that big of a deal.

Where is the proving grounds? I haven't found that yet.

You can get the same dmg from a katana, as a bastard sword, with a lighter weapon. The thing about dual wielding is that it gives you double the chances to score crits in a given round, and hence extra dmg. I do have a wm using a quarter staff in another mod, and he's pretty awesome too. Great axes are good as well, if you want to use two handed weapons. Personally, I'd rather go Barb/wm with great axe.
The proving grounds are on lvl 13, and be prepared, they are just that, proving grounds. You will find out what you need to do with your char there.

I have a fighter 10/Weaponmaster 30 who can solo the proving grounds. He uses long swords and has almost every single one in the game including the two +9 ones from the proving grounds. There are loads of nice longswords in the game to help you in those lower levels. Plus he can use a shild with it as well to make up for that AC.

Looks like I have to rethink my build for my titular characters. I should try to find that post of mine for that Ferromancer someone created in order to try to recreate my rogue wizard Story_01. It is hard to properly build one with the right blend of spells and strikes, so I think I will make him a weaker wizard this time and more in tune with the stealthy side of things. Besides, as long as I can use scrolls and wands, I should be all set. Perhaps get enough skills to make my own wands and use them at will at later levels? *ponders this proposition*