I think (single) grains as a category are some of the most overlooked whiskies. To my palate, the indies seem to have the edge here. As noted Hedonism is wonderful (I've only tried the first vatting). The Cadenhead Cambus 31 is spectacular and their Caledonian 21 is very good. Scott's North of Scotland '63 is also a treasure. Although, they are OK for a change, the OB grains like Invergordon 10, and Black Barrel are just alright IMHO.
I find the Irish single grains (from Cooley) to be quite good. Their OB Greenore includes aging in rum casks. It took some getting used to, but I quite like now. Limmerick bottled a CS (67.3%!), 9yo single cask that is also very interesting.
I think most single cask Canadian whiskies would technically qualify as single grains, although I've never seen them marketed that way. I love all I've tasted: Bush Pilot's Private Reserve 13yo, Hirsch 12yo (CS), Century Reserve 13 & 15yo.