The Axtinguisher looks pretty sick, and so do the Black Box and Wrench. If you meant best as in aesthetically, then that's my answer.

Any stock weapon.

Should've been stock only.

You can get all of them for free by installing a custom mod which replaces some warpaint ooor replaces just stock weapons with australium But firsrt variant is better.

They're all kind of ugly. Especially the stock ones and the ones that mix gold and wood.