If you're just looking to find the best gear definitely check all the traders in Mournhold, Deshaan.
Those usually have what I'm looking for, but will typically be the most expensive as well.

If Mournhold doesn't have what you're looking for Wayrest, Stormhaven.
I have found really good deals in Wayrest. A lot of the time they will have just as much gear as Mournhold, but I feel like the guilds selling there change a lot week to week. So it's not as reliable as Mournhold where you will find the same 4 or 5 guilds every week almost without fail.

If you are really trying to save gold and don't mind spending some time looking around Check Belcarth, Craglorn; Orsinium, Wrothgar; and Anvil and Kvatch, Gold Coast.

Also, I know that little town in Reapers march is really good, I just forget the name. I will also check Windhelm just because there are 5 traders right by the wayshrine, but those traders hardly ever have anything at all.

p.s.: If I'm looking for a set or materials that I know are usually expensive, I will look at every trader in the game. It goes surprisingly fast and you can find crazy deals. (I bought dreugh king jewelry, cuirass and greaves and only spent 50k)