I have had the 15 and it is (by design) drier and smokier than the 12 or the 18. All of HP is lightly peated but there is so much heather in the peat that it comes across very gentle and sweet. Nothing like the Islays.
The 15 is what I would call a narrow profile (others might say "one dimensional"). If you like the flavor of heather honey with a tinge of smoke, it is a winner. If you like a more complex profile then either the 12 or the 18 are solid choices. Both have the smoky honey but are also more floral and fruity. Both are very good IMO and worth what they cost. The 12 in particular is a usually a great value at around $40.