Cruising Alaska

I know there are a lot of you out there you have traveled to Alaska and cruised to Alaska. So I would like to draw upon your knowledge and get some opinions on a few items. One, what is the best month to cruise to Alaska, June, July or August?
Two, what cruise line would you take?
Third, what would you do in Denali National Park?
I know this is an RV site, but I am willing to bet a lot of you have traveled by cruise lines and have a lot of information.
Thanks