The following is a direct transcription of my scribblings on a notepad last night
Glenfiddich 12: Straw color. Sweet, almost bourbon-like aroma. Fairly typical Speyside. Not much peat, on the sweeter, lighter side. Not one of my all-time favorites, but not at all a bad whisky. Cinnamon, cardamom. Calvados-y notes. Splash of water doesn't do much- light enough already.
15: Slightly darker than 12. Aroma not entirely unlike cognac. Somehow thinner-bodied than 12. Also somehow peatier. Less sweet, with floral/vegetable notes. Splash of water brings out a fruitiness. More cinnamony. Despite thinner body much better overall than 12, gee whiz.
18: Golden. Aroma very much like cognac, hints of apple pie with cinnamon. Not a great deal of peat. Very full, rich. Discernible oak. Water brings out more spiciness- cinnamon and cardamom as with others, but hint of black pepper. Allspice? For more clear idea will have to drink other half of 50ml bottle when I haven't just had two fairly large drams immediately before. [Author's note: I am quite fond of the Patiala peg.]