Belkarth in Craglorn is the most active trading place for PC EU.

(Assuming best= variety & number of items)

As @TheCyberDruid said, the best (in terms of most sales & most active) is Craglorn. After that I'd say Mournhold is pretty good, other cities like Grathwood, Stormhold or Rawl'kha are pretty decent, and spots you should check when looking for items.

Have had guilds with traders in different cities; but from my experience you'll get good sales from all capital cities (Mournhold, Wayrest, Grathwood) along with Rawl'kha (for it's small and close bank+crafting hubs) and Belkarth (craglorn).

On PC EU, Tamriel Traders Guild (TTG) has the highest turnover. In terms of selling quickly, being on the front row of traders in Belkarth (Craglorn) is best. Another well-known guild is The Traveling Merchant, also on the front row. I am in 3 trading guilds. To give you an example of the average weekly turnover:

Tamriel Traders Guild, Belkarth, front row: 300 Million (highest on PC EU, according to them)
Pact Veteran Trade, Belkarth, back row: 100 Million
Redfur Trading, Windhelm: 10 Million

It takes much longer to sell anything in Windhelm. The best guilds have weekly sales requirements, 100K for TTG. This means they only keep active players who sell a decent amount. They also keep a trader in the same spot consistently. This has a big impact on the clientele. Based on my sales history, I can say that some people use the Craglorn front row as a one stop shop and don't even look for cheaper stuff on the back row.

If you want to sell / make gold regularly on PC EU, try to get into a guild in Belkarth (preferred), followed by the big 3 towns: Mournhold, Wayrest, Elden Root. Rawl Kha and Vivec City are also quite good. Vivec City is one of the best, if not the best, place for crafting writs, so lots of people go there. It is an expansion though, limiting the number of players who have access.