Yellowstone and/or Glacier National Park

I prefer the Tetons a million times over compared to Yellowstone. You get the wildlife and some of the best landscape in North America. Combine the for outstanding images. Check out some of Tom mangelsen work.

I'm about 20 min from west entrance to Glacier.
The landscape alone is beyond stunning. The wildlife is great as well but does take some work.
Sheep and goats are easily photographed within spitting distance. Bears are plentiful and Grizzlies show up often enough. Elk and deer prevalent. More Bald Eagles than I can count.

I've not visited Glacier, although it's certainly on my list.

I would agree with others that GTNP is better than YNP - but, if you haven't been to either, you should take the time to visit both.

A good area for wildlife, in GTNP, is (as others have stated) Oxbow Bend - also the back road between Moose Junction and Teton Village, lots of good foliage color there in the fall along with the wildlife.

In both parks, take two bodies. Keep a normal lens on one, telephoto on the other. You might see a Grizzly at 150 yards and around the next corner will be a bison walking next to your vehicle.

I was at Yellowstone once in April and got some great photos of bison, wolves, etc. in the snow. Summer is kind of crazy there; too many people.

Yellowstone offers a lot more opportunities with a variety of critters than the Tetons, but the landscapes in the Tetons are better. The thing is, in June or July it's going to be a stone witch with the crowds in either place. A black bear in Yellowstone in the high season of June or July can cause a traffic jam a mile long. And the Rangers are a LOT stricter and less friendly in the summer around bears or wolves because they have to deal with so many people and they just don't have time to give anyone the benefit of the doubt.
Glacier in July is not nearly as crowded as Yellowstone or Grand Teton. The traffic can be bad, but the places where you'll see wildlife won't necessarily be on the road. You won't get the bear shots you could get at Yellowstone or Teton, but there are some awesome bighorn and mountain goat shots to be had.
If I were to give you advice, I would say visit Yellowstone or Teton in the Spring or Fall. September is a good month to go, just after school starts, if you can get away at all then. It's still warm, the crowds are way down and the animals should still be as active as they are in the Summer.