This is going to depend largely on what is important to you. If you are arriving by cruise ship it will be determined by what else you have planned for the day and when your ship arrives. Early mornings can be nice because being the first out means there will be less people on the trails and is often the best time for wildlife. Evenings are often the best time for photography because you may catch the golden hour.
Overall, every hour of the day in Southeast Alaska is beautiful to witness.