Aside from the holiday festival around this time of year, EPCOT holds three distinct festivals but deciding which one should be on your list could be a problem if you don't get to do all three.
The three festivals are the Food and Wine Festival, Flower and Garden Festival, and Festival of the Arts. Each brings a unique atmosphere to World Showcase and soon to World Celebration as we should likely see kiosk expansions with the opening of the Showcase Garden area.
For AP holders, choosing a festival is pretty easy. Most will attend all three but for guests who don't have that luxury, choosing which festival to attend, if any, could be a daunting task.
The question you have to ask yourself is what is important to you? What will you enjoy more and what is your travel schedule?
The shortest festival is the Festival of the Arts which begins in mid-January and ends mid-February. It is a little over a month-long in terms of days. This year, the festival starts on January 12th and ends on February 19th.
The Flower and Garden Festival will arrive a week after the art festival ends. This year, it will begin on February 28th. It will run through July giving guests a lot of time to visit the park and take in the flowers and topiaries.
Finally, the Food and Wine Festival will begin at the end of July. No date has been announced yet. It will run through the month of November and give way to the final "festival" of the year, Festival of the Holidays.
I didn't include the holiday festival on this list for two reasons. One, I have not been to it, and two, most guests avoid it due to the traffic and time of year.
Making your decision regarding the festival you should attend, if only one or two, comes down to taste, almost literally. Here is where we rank the festivals in several categories.