I don't normally answer such questions. But this one comes up in my email so much—especially in the summer months—I'll give you a short answer in hopes that's enough ;~).
Yellowstone, like Denali, is a bit bipolar in photographic opportunities. On the one hand you have large geographic features that you can approach closely, so a good wide angle zoom is handy. On the other hand you have wildlife that may be some distance from you (and in some cases like bears, should be a long distance away if you're following park regulations). There you need a fast telephoto lens (fast to remove distracting backgrounds, telephoto to bring the animal in close). Of course, most of you reading this probably have your family along, so a mid-range zoom or primes to take "family vacation photos" is probably going to keep you out of the doghouse. In spring (which is usually a bit late in Yellowstone), a macro lens is useful for flowers.
But the real answer is simpler: you take lenses that match your shooting style, always. So many folk think that "a different lens" is the answer to any photographic problem. It generally isn't. One of the reasons why I'll answer this question here is that you're in a national park. If you want to get any reasonable shot of wild animals without violating park regulations, you need a long telephoto lens.
