OK - a little late to the party, but my vote goes to...
Two months ago it would have been Ardbeg without a question, but I was surprisingly underwhelmed with the beastie...
Plus the real truth is that I love all three of the big Islays, so since Laphroaig was lacking votes compared to Ardbeg and Lagavulin, I had to bump them up!
Anyway, each of the big three plus Bruichladdich has a special place in my heart (i have yet to have PE):
Lagavulin 16 was simply the first whisky that i bought that REALLY made me understand how good "GOOD" whisky can be. Now, I own the 16, a few 12CSs and the PX and they are all amazing!
Ardbeg: well, it is Ardbeg and generally amazing. Oogie is absolutely great. Ten fits almost every occasion perfectly. Provenance is simply the best whisky that I have ever had the fortune of trying. However, the 17 - while very very good - didn't completely blow me away, and the Beastie was frankly a bit of a disappointment.
Laphroaig: the 10 year is really good, but typically I'll take the Laga 16 or the Ardbeg Ten over Laph 10 most times. However, the 15 is simply the best Islay value bar none - at $55-60 / bottle, it is simply mind blowing good. Same with the QC (which i prefer more than the CS). And the 30 is something special: perhaps a bit muted compared to other Laphroaigs, but considering it's heritage, the smoothness is simply amazing.
Now I have to throw Bruichladdich in there too. While the 10 yr is strictly OK in my books (and not worth the money), the 15 is quite yummie. Also, the 14 yr links edition that i have (Turnberry) is quite tasty as well though it's not the best whisky ever, but it certainly is very easy to drink (and drink it I have!). Moine Mhor: ok - this gets the distinction of being the first whisky that I've bought a case of (followed by the Laph QC and 15). Why the Moine Mhor? I don't know, but something about it is absolutely fantastic. It is just peat plain and simple. Never have I gone through a bottle of whisky so fast. So, Bruichladdich - inconsistent perhaps, but when they're on, they are ON!