Even the cheapest in Alaska is pretty good.
So the key becomes the itinerary. Look over which goes where, and what you would like to see. Juneau, Skagway and Ketchikan are pretty standard. Some ships also go to Sitka and/or Icy Strait Point. Sitka's a cute Russian town with lots to do. ISP is a new port, so I don't know much about it except that the whale watching is good.
Some ships do Glacier Bay, which has some glaciers obviously, some do College Fjord, which has 16 (?) glaciers, some do Hubbard Glacier, which is gigantic, some do Tracy Arm, which has the twin Sawyer glaciers and is pretty. I've been to Hubbard and College Fjord, and am going to Glacier Bay soon, so I'll be able to compare, but most say Glacier Bay is a must.
So when you figure out what you want to see, it's easier to narrow down the ships that go there. Celebrity, HAL, Princess, RCCL and NCL cruise there, as well as one Carnival ship, Carnival Spirit, which is mine.
I like the fact that it has reasonably priced balcony cabins. I love having a balcony in Alaska, and I use it all the time. Some (Karen for instance) like to be on deck instead as well as saving money for more Alaska cruises, but I wouldn't do it without a balcony.
So... narrow down where you'd like to go and we can help narrow down the ships.