Definitely the Harbinger. I play on the Jedi Covenant server, but made a character on Harbinger to check GTN prices and there are a lot more people on the Harbinger than Covenant. I know the Ebon Hawk and Shadowlands are also popular servers, but I haven't actually made a character on them. Plus, the Harbinger GTN prices are much cheaper ($70,000 compared to $500,000 for example) and have a lot more items for sale. I'm seriously considering transferring my characters to the Harbinger server.