March to April: good night hours available, little bit less rainy, cheaper prices, Iceland is less touristic and somewhat more fascinating. Temperatures are cold but acceptable. Road condition not that bad. However many trekking areas are still closed (F-roads are closed. Read the Note below for explanation about what an F-road is), and ice caves cannot be visited anymore
August to September: a little bit less night hours, maybe a little bit more expensive than March, more crowded, less cold temperature, better road conditions, and more trekking area opened.
November to February: a little bit more rainy, colder temperature, more trekking areas closed, maybe road condition could be harsh, but with the great advantage that you can visit ice caves.
October: It should be cheaper, less crowded, a little bit more rainy, trekking areas begin to be closed, road conditions should be still good, and there’s no chance to visit ice caves.