Check out other thread under 'Cheaper Islay Malts' - I have tried most of the supermarket offerings. They offer tremendous value and I use them as my hip-flask malt.
The best is Sainsbury's as agreed by both Jim Murray's Whisky Bible (90 points) and it won a gold award in the latest International Wines & Spirits Challenge (see link for panel tasting notes):
http://www.iwsc.net/productPage.cfm?Pag ... ductID=850
M&S bottle says its from Burns Stewart suggesting Bunny, but tasting note on bottle suggests much more peaty so I suspect Laphroaig - M&S price currently £4 off.
As I say they are all exceptionally good value for money - only drawback is that that they are all 40% ABV, coloured and mostly vatted so change frequently. My guess is that the Sainsbury's uses less colouring than the others and hence more natural flavour - it certainly is lighter in colour in the glass. I suspect there is some Ardbeg and/or Caol Ila in this one.
Let us know which one you buy and what you think.