You will get lots of opinions as to “best”. I have taken 3 cruises to Alaska (Princess, Royal Caribbean & Carnival) and believe the itinerary and times in port are more important than the specific ship or cruise line. I would start by reading trip reports / reviews on this and other sites to see what might be if interest to you in the ports, then try to find an itinerary that spends the most time in those ports.
A one-way itinerary between Vancouver and Alaska will almost always give you more time in ports (and possibly an extra port or glacier day) than the roundtrip cruises from Seattle. Another upside to the one-way itinerary is that you can add a land tour in Alaska before or after your cruise and also spend a few days in Vancouver before or after the cruise.
Glacier Bay is a highlight for many and is primarily visited by Princes & Holland America with some Norwegian Cruise Line itineraries as well. Hubbard Glacier is impressive and visited by several cruise lines. Tracy Arm Fjord with Sawyer Glacier and Endicott Arm with Dawes Glacier are visited by many cruise lines, but ships can’t always get close to the glaciers due to ice. The small boat excursions from the ship are excellent for a better experience.
For us, the ship is primarily used for breakfast / dinner, sleeping and transportation in Alaska, so we don’t need the latest / greatest / largest ship. We rarely go to any entertainment / activities on a cruise to Alaska and prefer to watch the beautiful scenery.
I have cruised Alaska with and without a balcony. A balcony is nice to have, but I would not choose a balcony at the expense of not being able to do some excursions I was interested in. There are plenty of places inside and outside to relax and observe the scenery and possibly wildlife. Being outside on an upper deck gives you the ability to quickly move from one side to the other if whales or other wildlife is spotted.
Excursions in Alaska are expensive compared to what many are used to in the Caribbean, especially anything involving flights. Read reviews and watch YouTube videos to find those you want to do. Money is always a consideration, but I would recommend not skipping something you really want to do just to save a few hundred dollars. You will want to go back and do it eventually and the cost in both time and money to get back to Alaska will be much more than you saved by skipping it originally. The memories will last a lifetime.