What makes the best cider apple should really occupy you if you are planting an orchard from scratch and you want to make good cider. It makes sense to buy apple trees that produce fruit fit for purpose.
The reality is that you could make cider out of any apple (even Golden Delicious) and a large proportion of commercially produced ciders are made from everyday cooking apples such as Bramleys and all purpose apples like James Grieves.
But the best cider is made from cider apples which are generally fairly inedible. Cider apples are categorised by their combination of acid and tannin as follows:
|Sweet||Less than 0.2%||Less than 0.45%|
|Bittersweet||More than 0.2%||Less than 0.45%|
|Bittersharp||More than 0.2%||More than 0.45%|
|Sharp||Less than 0.2%||More than 0.45%|
As a contrast, consider the widely used Bramley:
|Bramley Apple||Less than 0.05%||More than 1.00%|
Without knowing what tannin and acid really do in cider, it is easy to see that cider made from Bramleys will be sweet (tannin is bitter) and very acidic. (For the interested amongst you we have a more detailed note on why cider apple tannin and acid are important)
The perfect cider apple is supposed to have a composition like this:
|Perfect Apple||Exactly 0.2%||Exactly 0.40%|
Kingston Black (the perfect cider apple) comes very close to this analysis. But life is not perfect and its yields tend to be low. Great for a couple of dozen bottles, not so hot for a few barrels.
So most of the other "best" cider apples are blends. Generally top notch cider is made using either a Bittersweet or Bittersharp as a base and then getting the acidity acidity right using Sharps or Sweets respectively.
The last ingredient in the best cider apple is described as "vintage". This means great (in the cider world) as opposed to old. Most of our cider varieties are "vintage" and you will find them described as such. Vintage cider apples tend to have the characteristic of fermenting less rapidly than the rest. One theory is that vintage cider apple trees do not absorb as much less nitrogen as their less classy cousins.
For more information on cider apple history, subtleties and esoteric varieties you will find "A Somerset Pomona" by Liz Kopas a good investment.