It really depends on when you're there and what you're hankering for. We always get to Epcot at rope drop so that we can head straight to Soarin'. We then do Test Track and/or Misssion Space and head back to Soarin' with FP tix. Because of that, we usually end up at Sunshine Seasons. Epcot's Future World doesn't really have great sit-down table service locations for breakfast. Mainly counter-service and snack options.
The World Showcase doesn't open up till later in the morning so if you can wait that long, then there are a lot of options over there. If you're hankering for a snack/light breakfast, Boulangerie-Patisserie in France or the bakery in Norway are great options. As a previous poster mentioned, Akershus's princess bfast is a very popular option for a sit-down. It can be expensive if you're not on the dining plan.