Pristine weapons in the depths. If you don't like the bonus roll then just drop it and come back next blood moon to see if the weapon/roll is better. If the weapon is good, but not the roll then you can take it to a Rock Ocktorok to suck up and reroll a bonus roll. They also repair weapons. Make sure you pick up the pristine weapons and drop them or else they won't change in the depths.

Gerudo weapons give bonus fuses.

I heard forrest dweller weapons are good to fuse plant stuff like puffshrooms, fire fruit, muddlebud. It's supposed to make you hot multiple times on the weapon without coming off.

If you have a construct weapon make sure you fuse it with a construct part. Those rollers after you kill the big block mini bosses work well. There are bonuses in this game that is eluded to in some item descriptions.