Guild Wars 2, easily. It has the second best F2P option (I would put it below FFXIV, but I should also stress that the difference between the two isn't big an preferences in gameplay and the like would be far bigger), the game was designed in such a way that a potato could run it if you aren't doing WvW (the mass PvP system), and if you do decide to buy it the fact that it has no subscription means that you can comparatively cheaply play it for as long as you want to (and don't have any pressure to "get your money's worth").
"A shouted order to do something of dubious morality with an unpredictable outcome?
Thweeet!
"
My FC is in my profile.