In my opinion the best food is definitely Panorama, just the other side of Spili. It's walking distance from the fountains; make sure you try the owner's mother's dolmadakia.
As you go into Spili the wonderful restaurant on the left covered in flowers is Maria Costas, large portions, and Costas does an excellent grill.
I also tried the one in the middle of town with a terrace overlooking the carpark - the food was poor and the service dire and we were ripped off, so wouldn't recommend that!
