For weapon preference I think it comes down to 2 things:
1. Damage + effects vs. opponent type, and
2. User finesse (a.k.a. *user* skill level) vs. weapon motion and range.
For mobs.. yes, the Blade of Death can be excellent.
The Blade of Death has excellent damage for a weapon, but its effect in general is often less useful (again, excepting low level mobs). Give me a No-Dachi of Winter *ANY* day over the the Blade of Death. Why? It simply has a *much* better effect against most opponents. The same can be said for the Sapphire Greatsword (which you can get earlier than the Blade of Death), and even the Diamondlight Greatsword (..where the various "strikes" can interrupt an opponents attack). Of course none of these alternatives have the same average damage.. but DEFENSIVELY they are vastly superior.
Then there is user skill and the weapon's physical range and speed, as well as its range of special attacks.
The special attacks for a single-handed weapon is the worst in the group.. by far. The best is Ninjitsu.. but it also requires the highest degree of user skill. Polearms (and even a lowly staff), has the second best. The third best is from two-weapon special attacks.
Which has the highest damage potential from a special attack?
A HEAVY POLEARM. Where you can often get damage (with a high strength) to around 50-100.
It is however slower, but not much - and if you test it out throughly (with a high degree of user skill for every weapon attack), you'll find that the overall "kill-rate" is pretty similar to Heavy weapons, Dual-Weapons, and Ninjitsu.. UNLESS you have another damage effect and are against low level opponents. (Note: one of the primary reasons why the kill-rate is fairly comparable with polearms is because you will often hit more opponents in a single attack maneuver, OR if against a single opponent - can hit for a higher rate of damage per attack.)
Adding in effects however:
The highest melee "kill-rate" (by far) is dual weapons katana (or No-Dachi) and wakizashi with both critical strike and inflict wounds. Offensively this makes the Hatamoto SUPREME (..the thing is a blender :mischief

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