From experience, I can say it truly varies.
As has been mentioned, against unarmored/poorly armored foes, swords, especially the longsword, are the best. Longswords are better than two-handed swords, as you will, in all likelihood, get no penalty for using it with a shield. Against armored foes, either high-quality longswords or a sort of axe are optimal; halberds are great for this, especially if you're in a scenario where slashing/hacking attacks don't do much no matter what, being a pike/axe fusion. If you just want pure severing goodness, great axes seem optimal, although battle axes don't fall too far behind, either. I still prefer halberds, since it's perfectly possible to sever large limbs by stabbing with it, even against heavily armored foes.
However, no matter what weapon you choose for your limb-removal, try for AT THE VERY LEAST bronze. Bronze is great against unarmored/poorly armored foes, as it is rather light but powerful. Iron is optimal for human-made weaponry, which means your longswords, great axes, and halberds are going to be iron. If you're using short swords, spears, or battle axes, it is reasonable to enter a dwarven fortress and take one.
I'm unsure about the quality those are; I'm also unsure if the iron weapons available in human mead halls are better than base-quality steel ones of the same type. If you're extremely lucky, mead halls have masterwork items; moderately lucky people find exceptional ones. You'll find a good amount of superior-quality ones there as well. Is a superior-quality iron longsword better or worse than a no-quality steel longsword?
I should test that.