Well, Iceland is a pretty reliable place - northern Norway also, and northern Canada, around Whitehorse or Yellowknife, or parts of Alaska. Between 60 degrees north and the arctic circle is the sweet spot. Best time of year is fall through winter, and beyond that it's down to solar activity and weather. As it happens, @chaseburnett authored an excellent guide just last night: https://steemit.com/photography/@chaseburnett/photographing-the-northern-lights-an-in-depth-guide