Twin Blades:Empty Skies or Heine's Invasion?
They both suck.
As Tri-Edge said, Spin Nezumi and Spin Drake are the strongest twin blades in the game, when fully alchemized to +5, and customized with the best possible items (Nil items, Sabertooth Tooth, Ginseng or Gurah Fist). Since they are non-Rarity 5 weapons, you can alchemize them without cheat codes, simply by having 6 Spin Drakes or 6 Spin Nezumis (or the long way...having 1 of them and 5 other Spin weapons alchemized to +5 already).
Broadsword:Deadly Fearblade, Silad or Heine's Shadow?
Silad is the strongest. Deadly Fearblade MIGHT be the one with the highest level, but its effects sucks badly. And by Redemption, Heine's Shadow is crap. And of course, Broad Dowan and Broad White alchemized and customized just like above are also the strongest Broadswords.
Scythe:Shadowy Death, Godess Scythe or Heine's Zero?
They're all the worst scythes you can ever equip to Haseo.
Scythe Hidaru, and any other non-Rarity 5 weapons Haseo can use of a higher level than Hidaru are the strongest scythes, making Scythe Dakini the strongest one overall. But well, Hidaru is the best looking IMO, so it's worth sacrifing a few attack points just for looks....and Hidaru is still way stronger than any of the weapons you mentioned anyhow.
DG:DG-Y, DG-Z or DG-0?
DG-Y: Allows for unlimited Skill Trigger usage while combined with the Super Charge thing. The best if what you want is to just spam skills all over without ever being touched and only ending the massacre when all enemies have been killed.
DG-Z: Has the "strongest" default attack of all guns, and can pretty much kill enemies with just one critical hit. The best if what you want to do is just shoot all over like crazy.
DG-0: Works only if you have a very low HP, but can inflict the strongest damage out of every other weapon in the entire game. The best if you wish to have the strongest power ever, and have fun with what would be a rapid-fire shotgun that blows brains out.
And to shorten things up:
Rarity 5 weapons are the worst weapons you can have, including Empty Skies and the Doppel weapons (and some Lost Weapons), and if what you want is to really use the strongest weapons, then use the highest-level non-Rarity 5 weapons, alchemize them, and add some of the items i mentioned. You'll soon notice the difference.