At the top of my list of edible mushrooms that I have not tried is Agrocybe parasitica. That is because it is a large mushroom and can occur in fairly large clusters. It should also be possible to cultivate this mushroom.
Above picture is courtesy of Reiner
Various sources disagree with respect to the edibility of this mushroom. None list it as poisonous, but some advise caution. On the other hand, Watling and Taylor (1987) describe it as an excellent edible. Their description can be found here.