The weakest weapon skills that come to mind are the following:
- Redemption's "Quietus"
- Rhongomiant's "Camlann's Torment"
- Farsha's "Cloudsplitter"
- Caladbolg's "Torcleaver"
and just about all relic and mythic weapon skills need the refinements.
All of the favorable empyrian weapon skills have a great damage modifier, a chance at critical hits or both. These come to mind:
- Ukonvasara's "Ukko's Fury"
- Verathragna's "Victory Smite"
- Almace's "Chant du Cygne"
- Kannagi's "Blade: Hi"
- Masamune's "Tachi: Fudo"
- Gandiva's "Jishnu's Radiance"
(Rudra's Storm and Wildfire I am unsure of.)