For me it has switched so many times. Started with RF Online as my first ever. Played it to level 45 of 55 (at the current time). Then the company just suddenly dropped it. Years later it was restarted (twice since it happened again a year later).

Then it moved to the main few I played longer. Vindictus (which I still play today). Then there's the ones I'll go back to that I played temporarily when bored S4 League, League of Lengends (which I only find good when you play with friends...otherwise I get bored of it). Tera was good, but with so much detail to it my computer could barely run it at low graphics....now whenever I get a new laptop I may go back. Other than those I do play Final Fantasy XIV still (well...Will go back after I get a new computer set up). Dunno what it is about it, but I keep getting pulled back. It even hinted at the FfVII remake before it was announced.

There might be more, but it's been since 2006 since I've played the occasional few mmos I've tried over the years and mostly dropped. Today it's just Vindictus and FFXIV really