Best ever is a hard choice, but each of my choices come from my first really successful fortress. Of the 83 artifacts produced over the 48 years, I'd narrow it down to...
4th:
Windyfly: a Golden chain Leggings worth 211,200
Mostly for humorous appearance. I imagine them less like chain mail and more like shiny parachute pants. I have (unsuccessfully so far) tried to pair them with a hammerdwarf so that I could create MC Hammerdwarf.
3rd:
Choppedfuture the Familial Bitterness an Aluminum anvil worth 580,800
A very useful anvil! I was much more settled by this point, having survived a number of sieges and having a decent military and numerous traps. Still, the most expensive item produced. Popular choice for images to engrave on later artifacts.
A close 2nd:
Foughtaces the Misty Gulf a Zinc anvil worth 27,600
This was the first artifact my fortress ever produced, and did so in the first year or so I think. It was extremely useful to have a second anvil so early on and I credit it as one of the biggest items that led to the success of my fortress.
But no, the best artifact I have ever had come from my fortress would have to be this one, which came in just a handful of artifacts after the Zinc anvil, but before the golden leggings:
1st:
The Birds of Diversion a Platinum Left Gauntlet worth 199,200
It is encrusted with Brown Zircon, Studded with Platinum decorated with fox leather and goblin bone and encircled with bands of turtle bone and Gabbro. This object menaces with spikes of Platinum, fox leather, and Gabbro.
Shortly after it's creation, it was claimed by a Dwarf who would become my greatest champion: Medtob Workpowers The Blameless Purge of Celebration. Wielding his dual swords, and wearing his Platinum left Gauntlet, he has slaughtered 251 people: Goblins, Humans, Elves, Dwarves, and Inira Fireysparks the Sweltering Diamond of Jewels the Dragon. Oh, and one horse.
It's covered in blood, vomit, human blood spatterings, goblin blood, horse blood spatterings, mud, and kobold blood spatterings.