I was born in Iceland, but currently live in Denmark, I really feel that anytime a year is good for visiting Iceland. I went to Iceland one summer and I got all blue sky and sunshine the whole trip, I was hoping for a more moody dramatic scene and was not very happy with my photographs. After a second look, I am very happy with them
(the last photos) . Iceland during the winter is magical, but its sometimes tough to be creative when you are cold.
So my advice is go when you feel like it, anytime a year is good time for photographers.