Try and get slash or blunt weapons if possible. Shadowknights have less max piercing skill than the other two. Something like Iceflame may beat out your current weapon since it has a 0.5 better ratio and a proc, but maybe not. I haven't done parsing to see how much the lower max pierce affects DPS.
For players on the blue server:
An Ebon mace would be the easiest upgrade for now, as Tycohunden says. They are only like 1-1.5k now, and a higher ratio than your current weapon. You could also use a shield at that point for a bit of extra tank.
If you are set on using 2handed weapons, the cheapest upgrade would probably be one of the lower quality Velium weapons from Dragon Necropolis, such as
https://wiki.project1999.com/Forged_Velium_Claidhmore . That is also around the same price as Ebon Mace, and higher ratio. Only issue with these weapons is they can be harder to find in tunnel.
Reaver is obviously great as Darkreap mentions, but much more expensive that the weapons mentioned above and harder to sell.