The UI is the point of interaction between the player and the rest of an MMO. We don’t really think about it that way, but in no small part the UI is the game. Sure, it becomes more obvious in some games than others – most serious gameplay in EVE Online has you zoomed so far out that you don’t really see the graphics and you’re just clicking through windows – but every game involves a lot of glancing at your action bars, checking various gauges and maps and indicators, and altering your play accordingly.
With all that in mind, think about your UI of choice. Every UI has its strengths and weaknesses. Some of them are good at showing off important information but have it laid out in a counterintuitive way, some lack much in the way of customization options, some just are always going to bother you a little no matter what you do. So which MMO has the best UI? The most pleasing, most comfortable, and most efficient at displaying relevant information?