Glengoyne 18 Year sits right in the middle of their range, between the 10, 12, and 15 and the 21, 25, and special releases above. In keeping with the trend through that range, it is significantly more sherried than the expressions below it. While I can't find definite information, I've seen suggestions that it is aged entirely in refill- and first-fill sherry casks, with a significant proportion of the latter. Like the 10 Year, I purchased this miniature while I was in Scotland in 2013.
It is bottled at 43%, without coloring but with chill filtration.
Glengoyne 18 Year
Nose: strongly sherried, a little dank, stewed fruit, savory, moderate American/European oak, vanilla/cacao, herbal/grassy, malt in the background. After adding a few drops of water it is more or less the same, but somewhat diminished except for amplified vanilla and some floral notes come out.
Taste: alcohol reads as stronger than 43% - strong sherry up front, joined by oak tannins around the middle, reduced sherry near the back with more noticeable malt. After dilution it is sweeter up front, with less sherry and more savory oak near the back.
Finish: oak spices, savory notes, polished oak, orange peel
Much like the shift from Tomatin 12 Year to Tomatin 18 Year, this feels like an entirely different whisky than the 10 Year. It has more in common with the defunct Glengoyne 17 Year, which was also very sherry-driven. I'm not sure if it's about the casks or the spirit, but the finish on this whisky reads a lot like Ben Nevis to me, with the peculiar twist of savory oak at the end. That would put it a bit ahead of Tomatin 18 Year, which is the most comparable whisky I can think of, assuming they're roughly the same price. In the States, where Tomatin tends to be significantly cheaper than Glengoyne, I'd give the nod to the latter.