I expect you mean Server as opposed to sever (to dismember/cut off).
The EVE Online server cluster is often referred to as one server. It is actually 12 IBM blade servers racks with 280 total cores, and more than 4.8 terabytes of local storage.
(source: EVE Online | EVE Insider | Dev Blog)
Each solar system has its own sub-server though (a cpu core and some memory), but when you move around space you won't notice these transitions between server nodes.
As a result, there is no "most popular server" because of the appearance that there is just one server.
The most popular solar system is Jita, which is home to a massive trade hub in high-security space. On occasion the number of players in this one system are counted in the thousands. Most of these players are trading their items, while others are simply waiting for you to undock into space with your newly aquired shinies so they can pounce you.