Depends who you are fighting. The Void Lord is weak against Spirit so Paladin's Longsword is the tool for that job.
Nanocarbon Sword does more damage and Electricity damage, Shadow Blade is second in power and does Death damage, in third place is Melporth's Sword dealing Toxic damage. The thing is almost nothing in this game is vulnerable to Death magic, and a huge range of creatures resist it to various degrees. Many more creatures are vulnerable to Electricity.
You can look up the creature resistances but it seems to me they're thrown around so randomly it's barely worth bothering with unless you have a difficult boss battle.
I'm using 2h weapons, but I find it useful to keep 1 of each of the following elements: Spirit, Fire, and Lightning. Most enemies you'll encounter are weak to one of these and using the right element for your target is often a better bet than opting for raw dps output, ignoring resists.
That said, there are two major considerations besides resists and weaknesses. The nanocarbon sword tops the weapon dps chart by a large margin... But the Paralysis attribute of the shadow blade is an awesome defensive perk, and particularly so on a weapon with such a high attack speed.
Resists not withstanding, I'd use the nanocarbon sword to speed clear most any area. The shadow blade, however, would be my go to weapon for anything that gave me any sort of trouble. The RNG paralysis procs aren't something you can rely on in any single instance, but if you can reload saves and try again, it will eventually secure you a victory over almost anything. RNG is never to be underestimated - in either direction.