For me the best in the range in terms of value for money. Still has most of the signature Glengoyne notes with a slight sherried edge to it. Not as bitter as the 18 or 21 but not as sweet and lowland-like as the 10 and 12. The toffee in the 15 is pronounced with honey, green apples, dried apricots and some red fruits with no note completely dominating over the others. Finish is medium with little bitterness, more spices.
The main flaw is how thin tasting this is (unfortunately this is true for the 10, 12, 15, 18 and even the 21 to some extent) If the ABV was increased to 46%(minimum!) I think my mark for the entire range would increase by a few points