While it's somewhat build dependant, they best one for just about anyone (Especially a non-specialist like Nightblade) is the Gauldur Amulet, because it enhances all 3 Stats. If you're a Mage, then the leveled Savos' Amulet is better (Up to 70 magicka) I also run into necklaces of Poison/Disease immunity right regler once I hit level 30, give or take. If you happen to be a redguard Werewolf in the Savior's hide, whoopty funkin' dew, but for the rest of us, it's kinda nice not having to shoot down hawks whenever you kill a Vampire, or guzzle potions surrounded by mixed Falmer, and Charus.
I don't like the Necromancer's Amulet, nor anything (Like Vampirism) with more penalty than bonus when the bonus isn't that hot to begin with. Fortify Conjuration? great, almost all of them are duration spells, so you don't have to spam them faster than your Magicka can recover, and in return, you don't recover as quickly? Sign me up! [/sarcasm] Even if you're not going to wear the Gauldur Amulet the whole game, you can get it at low lvel, and have an extra 90 stat points when you need them most. If it's not THE best amulet for your build du jour, it's most likely a close second, and almost certainly in the top 3. Because of this mass appeal, across all possible builds, I'd say the Gauldur is the best amulet in the game.