Morels. These are not the white, button mushrooms you grew up with. As far as mushrooms go, the ne plus ultra has to be the morel. If you can't pick them yourself, they are available at many farmers markets in spring. While they're pricey, it's really worth picking up a few.
The best and simplest way to taste all the mushroomy, earthy goodness? Simply fried in butter until they get a little bit crunchy around the edges.