There isn't one best. It depends what you want. Morganthe has them with stats and blades. The Darkmoor treasure box and the Aquilla bosses have mastery with stats. Shane von Shane in Upper Halls has stats and shadow shield. Most of the Darkmoor bosses have stats with no cards. Divine influence can be good too. It all comes down to your needs.
The best amulet I have gotten gave my the hydra spell. I don't remember the name of the amulet but I know I just bought it from the bazaar. They have a lot of good, cheap amulets at the bazaar.
