Meph in all difficulties is a decent target. Diablo and Baal less so since you need to clear Chaos Sanctuary (ouch) and the five waves of minions for them. Andariel (in Nightmare and Hell) is an option if you desperate want unique jewelry, but she can only drop up to SoJ/The Mahim-Oak Curio in NM and Carrion Wind/Mara's Kaleidoscope in Hell. She also cannot drop any Rainbow Facets. She can drop all set jewelries even in NM though if you care enough about it.
In Hell, Eldritch the Rectifier, Thresh Socket (if your WP in Arreat Plateau is near the entrance to Glacial Trails), and especially Pindleskin are also a good option if you haven't activated the Hall of Pains waypoint yet. In fact, between Hell Mephisto and Hell Pindleskin, you can find every single unique (and set) in the game except Tyrael's Might (which is more of a trophy than something one would actually use IMHO), of which you'll have to either run Baal or alvl85 areas for. --- If declaring my opinion is akin to declaring war, then war I shall declare on the so-called veterans.