been to all three, although my favorite is Glacier, i think EVERYoNE should see yellowstone, it is like no other place. moonscapes, geyers, buffalo, bears, etc. it will be crowded. i didn't think i'd be too thrilled about mammoth hot springs but it was one of my favorite spots there. there are some lesser used hiking trails that can take you past thermal features to hidden lakes in the woods. As for yosemite, busy busy busy. think about going to banff and jasper sometime too, the canadian rockies are spectacular.