It depends. I'm not loyal to any one cruise line. Usually, I'm looking for a cruise with:
1. Low single supplement. Unobstructed balcony is a must
2. Good itinerary, including long days in port. Preferably, four port stops.
3, Glacier Bay is certainly a plus, but not a necessity. For first time cruisers, though, I would say a must.
4. A one-way north to south ending in Vancouver, I love the last at sea day heading towards Vancouver and then sunrise the next day coming into Vancouver Harbor. Can be stunning.
5. I like to embark in Seward. That gives the opportunity to take the train down from Anchorage, arriving in time to take 6-hour boat tour of Kenai Fjords. Upon return from the boat trip, I head over to the cruise dock to start my seven days heading to Vancouver.
6. Having Sitka and/or Icy Strait on the itinerary is a plus.
You may notice that I haven't mentioned the feature and amenities of the ship itself. It's not that they aren't important, but for me, Alaska is about the scenery and photography opportunities much more than fun on the ship. One thing I do appreciate on the ship is the ability to find a quiet public area to read. Holland's old Explorer's Lounge was a good example. Unfortunately, finding such a spot these days seems to be harder and harder. I don't need to have music blaring everywhere on the ship.
As far as food and entertainment, I know that meals will be good to very good no matter what cruise line. I'll drop in on a show once in a while, but they aren't a big part of my cruise experience. I did enjoy Holland America's BBC Earth Concert on last year's cruise.
That's about all I can think to regurgitate on my thought process for selecting a cruise line and itinerary. And, of course, YMMV.