Grayswandir is generally agreed to be the most powerful artifact weapon in NetHack. It is a silver sabre, which is by itself a solid weapon. What makes Grayswandir especially noteworthy is that it deals double damage, to absolutely everything. Its base damage, along with its enchantment and its wielder's strength bonus, are all doubled. On top of this, since it's silver it deals an extra 1d20 damage versus enemies like demons and undead that hate silver. This makes it nearly twice as damaging to demons as Demonbane, and even better against undead than Sunsword - while all at once being one-handed, and generally effective against everything. As a nifty bonus, it supresses hallucination while wielded.
Grayswandir is so good, that almost no character wants to wield anything else if it's available. The downside is that it's lawful, so neutral and chaotic characters will get blasted when they pick it up, or wield it. In many cases, the blast damage is well worth it, and if you're lawful, it's almost good enough to warrant blowing a wish. It's certainly worth it if you already have most of an ascension kit.
Since it's a one-handed weapon, the classes which can #twoweapon ought to do so. Another silver saber, or even a silver dagger or silver spear is a great offhander when fighting through the hordes of demons in Gehennom. Outside of Gehennom, and on the ascension run, a katana is a phenomenal choice if you can find one. Failing that, a longsword works very well, too. In Slash'EM, you can use another artifact. This is pretty much an open invitation to wield Snickersnee or Excalibur.