What is the best weapon from the boss soul of the Fume Knight?

[Arthur ¯\_(ツ)_/¯]

[Vaednari]

In my experience? Absolutely his greatsword.

[TrueArchery]

Actually, this is hard to answer. Because both are really good.

The Fume Sword has the longest Straight Sword reach and a beautiful moveset coupled with innate Dark damage. It's suited for a Dex-build. If you run like 20+ in both Int and Fth, it's worth it to infuse it with Dark.
It can be buffed with Dark Resins and the Hexes Dark Weapon and Resonant Weapon. I'd go with Resins with essentially no Atn-, Int- and Fth-investment and with Dark Weapon in case that you run a hexer build. Resonant Weapon is really ineffective/a VERY endgame thing.
The easier to use weapon for sure.

The FUGS is one of the most damaging, pure physical weapons, but is very stat-demanding. 40 Str and 30 Dex plus it's one of the heaviest weapons out there.
It cannot be buffed and infusing it isn't very smart.
It has special traits: It doubles as a really good shield and can deflect weak attacks, similar to Greatshields. It also has some attacks which block while you attack, so that goes hand in hand.
Since it's an Ultra, it'd normally be really bad competitive-wise (although those things just matter in high skill level play tbh), but it's got a glitch which makes it top tier, essentially what it does is deflecting almost all kinds of attacks with NO stamina consumption and it guarantees you a free followup attack. The attacks which aren't affected are so slow that they can be reaction-punished with the already mentioned built-in blocking-attack.
The skill required to use this properly is high, very high, but obviously rewarding because the weapon will just be broken as hell. Technically better, but hard to use and a "one-trick-pony".
Since it has very high AR, it's also good to keep in your pocket for backstab-fishing, guardbreak-ripostes and parry-ripostes as it'll oneshot or severely damage anyone getting hit.

[King Robert Baratheon]

Originally posted by TrueArchery:

it's got a glitch which makes it top tier, essentially what it does is deflecting almost all kinds of attacks with NO stamina consumption and it guarantees you a free followup attack.

Oroboro did a video on this. I'm too lazy to try to execute the glitch(es) but it looks pretty cool.

https://www.youtube.com/watch?v=KDame7qv_Tg