Cask Strength: Choose this if you want peat, smoke, heat, strength, and oomph.
Quarter Cask: Choose this if you want pungent peat, youthful exuberance, and bold assertiveness.
15yo: Choose this if you want something gloriously complex, smooth, refined, silky, sweet, peaty, rich and satisfying.
Yes, they are all different, but they're all equally good.
As you can gather from my thoughts above, I feel each one caters for a particular mood or textural desire, so choose one based on
what takes your fancy.