We are going in January because that is the only time I could get my in-laws to watch our kids. I know it isn't the ideal time of year, but at the very end of January, you get about 6 1/2 hours of daylight. It is also a great time to see the Northern Lights.
The prices in the winter are also great. We got hotel, airfare, Blue Lagoon tickets, and a northern lights boat tour for only $679 a person. That is for three nights and five days (arrive early in the morning the first day and leave late on the last day).