Rawl'ka in Reaper's March tends to be the #1 spot to hang out. (I don't log out there anymore however as it can get so busy you have trouble logging back in).
Craglorn used to be decent, I still check it sometimes, but the other Undaunted Enclave cities: Grahtwood, Mournhold and Wayrest are pretty good as well. I always check them as well when looking. Riften has a couple good stores too (they should really move all the kiosks for riften inside the city, close to the shrine, that city would be almost as popular as Rawl'ka if they did, I bet).
Elden Root in Grahtwood, Wayrest in Stormhaven, Mournhold in Dashaan, and Belkarth in Craglorn have a cluster of seven traders. These are usually the best place to hunt for something hard to find, and likely to have better stable prices since they compete with each other.
The 12 remaining alliance zones will have a cluster of 5 traders in one of the major cities. The Hallow City in Coldharbour has a 4 cluster and Mistral in Khenarthi's Roost has a 2 cluster. Aside from Khenarthi's, all the remaing zones should have 1-2 solo traders near a wayshrine as does each outlaw refuge. Solo traders are likely to have more varied prices since there's no one near by to compare against. You might find an amazing deal, or get ripped off.
Personally Mournhold is may favorite, 7 shops circling the wayshrine.
All the shops and crafting stations circle right around the Wayshrine, leaving easy access to everything. The only bad thing about it is that the bank and stables are on the other side of town, but it's not a long walk.