Best single-malt Scotch...

You all need to join the Scotch Malt Whisky Society, if you are at all serious about Scotch. The commercial bottlings listed above are all good, but the best bottles I have had were single malts which were single-cask, undiluted, and not chill filtererd. I suppose my all-time favorite was a 16 year old Ben Nevis, but can not remember the exact bottle number. Some over-30-year olds (which I collect, up to a point - I wouldn't want to die with an undrunk collection) I have had contained bottled happiness. I don't know how they capture that, but some bottles have it.

Curriemt11 mentions the 1966 Springbank Local Barley. I drank my bottle of it, and it was wonderful - I would certainly rank it above anything else mentioned here. Little did I know it would be worth thousands, but I buy the stuff to drink, not admire or keep or resell. I have had other Springbank bottles which I liked more.

I agree with the comments above about age. I feel that 16 -24 years is generally ideal, although some, lowland scotches for instance, are generally regarded as best drunk at younger ages. But once in a while an over-30 bottle has something magical that I have never experienced from a younger bottle. I have had two over-30's which were not excellent - you pays your money and you takes your chances.

For some reason young scotches are popular in Italy, I can't imagine why.

I am entirely persuaded that avoiding chill filtering is important to tasting the full flavor of a whisky.

45 year old Springbank! I volunteer to help! Rare, and likely to be very different and interesting, but not necessarily "better" than other Springbanks. Scotch really does not improve past the early 30's, as far as I know. But I intend to continue the research...

I agree with whoever above observed that the oak McAllans are not comparable to their traditional format, but the sherry in McAllan is sometimes just too much. You need to keep a few bottles of different types handy!

I will put in votes for the great Glenmorangie and Balvenie bottles, first rank distilleries. They are very rarely available in independent bottlings.

"improvement" is a value judgment. lengthy aging changes scotch, which can become very dark and can have unusual, intense flavors, often unlike what we normally think of as Scotch. you should try them for yourself. If you are ever in Edinburgh, go by the Vaults at Leith and try some Society undiluted, non-chill filtered, single malt, single-cask whiskys. Also, the Society has touring tastings in major cities in the U.S.
I consider Michael Jackson's guidebook more useful.