The Prototype Rancour is good for any build since you can't go wrong with the ATK stat. The only catch is that it has a Physical DMG Bonus as its secondary stat but that shouldn't matter much, since most sword users either rely on physical damage for DPS or aren't suitable for DPS at all.

Are prototypes good in Genshin impact?

Four-star Prototype Rancour has one of the highest base attack powers. It is a true DPS weapon with a physical damage bonus as its secondary stat, and an increased Base Attack and Defense of 4%-6% upon every hit. This effect will last for 6 seconds and has a maximum of 4 stacks.

What is the strongest weapon in Genshin impact?

If someone asks what is the most powerful weapon in Genshin Impact, then the correct answer would be this one: Wolf's Gravestone. It's a claymore that doesn't care how tough an enemy is because it will just keep stacking ATK buffs.