That depends on what you want to do and see.
Most people go to Alaska during the summer because they want to see wildlife and be there during high season when everything is open.
Summer is May through September with temperatures that are similar to Wisconsin, although in spring they are often warmer than us.
Along the coastline will be warmer and have more rain than the interior. You’ll see more waterfalls and rainbows too. Cruise ships sail from mid May to Labor day.
As you explore the higher elevations, you’ll notice cooler temperatures.
Springtime in Alaska offers gorgeous flora and fauna and it’s great for wildlife viewing. The animals are birthing, have plenty to eat and seem more active.
Summer has the midnight sun, salmon fishing and festivals.
Fall is known for it fall foliage and the possibility of seeing the northern lights.
Winter has less daylight, northern lights along with the Iditarod but many things like parks are closed. The train runs from Anchorage to Fairbanks just once a week.
Personally, I’ve been to Alaska in spring, summer and fall. Each trip was great in its own way but my personal favorite is springtime, early June. The wildlife is active, everything is green, the sun is shining and the tour guides & servers are still fresh.