Unique/Legendary items are mostly found in Paradise Valley, Hermits Isle, Alamos and Dragonsand.

Although you might find a few in Silver Cove and/or Darkmoor. Especially look in the dungeons in all of these areas.

Sweet Water is pretty barren of nice artifacts. But keep your eyes peeled anyhow.

IIRC, New Sorpigal resets every six months (game time ) and I think Castle Ironfist resets every game year. Could be off, it's been at least 5 years since I last played. But I played dozens and dozens of games during the 15-20 years that I did play.

Can't play MM6 (7 or 8) on my current computer. Somehow, when my rig was serviced about 5 years ago, the graphics card fan needed replacing and it had an adverse effect on the games' animations.

Shots freeze in the air (along with the whole game). So MM6 is unplayable to me right now. (SOB!!)

Anyhow, hope this helps. Don't be afraid to leave an area. The chests and game-placed items will still be there next time you come back … unless you wait waaaay too long. And still, there'll be new, just as nice, treasures awaiting you, with each reset.