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.
9 May 2021
It is 11 years since I made that post. This year has seen a flurry of fb posts from people in northern NSW describing this as the best tasting mushroom that they have ever eaten. It seems that it occurs in quite large outcrops in that area as well as in New Zealand. It also occurs in Tasmania. I am still to taste it. Oh, and it has had a name change to Cyclocybe parasitica.