what are the best stores?
Wonders of Thedas is definitely a top tier shop, it has great equipment for mages (Wintersbreath and Reapers Vestments among them).
Gorim in the Denerim market sells a few good items (Rock Knocker, Katriel's Grasp), but nothing special imo, the rest is generic dwarven stuff, but he has a pretty wide selection.
Herren in Wade's smithy is I think your best source for armor, he sells a full set of everything, and also the Felon's Coat and Evon's Mail, which are pretty good armors. Plus it is place where you can order the custom dragon armors.
Early on, the merchant in Lothering (the one at the chantry that acts like jerk to people) is a pretty good source of generic equipment, at least his stock beats Barlin's for sure. But again, you can acquire some pretty decent equipment during the Lothering quest, so he is on the bottom end of the list.
Garin in Orzammar sells the Lifegiver ring and the Rosethorn dagger but the rest of his stock is pretty generic, so I guess he's among the honorable mentions; as is Legnar, the other dwarf merchant in Orzammar, who sells the Shadow of the Empire armor.
Alimar in Orzammar sells the Effort boots, which are part of a great armor set, but again, the rest of his stock is average, so honorable mention again.
Ruck in the Deep Roads has some pretty good uniques, and his shop is a good place to unload the tons of loot you've gathered during the search for Branka quest, so I think he deserves a decent spot on this list.
Another honorable mention is Mikhael Dryden at Soldier's Keep, as he forges Starfang and sells uniquely colored variants of armors.
Also, I guess we can't forget about Bodhan, as he is the camp merchant and has some good stuff like the Spellward.
Varathorn is the ultimate source for crafting material with his unlimited stock of virtually everything, so for that alone, he deserves a spot.