Apples picked and eaten straight from the tree (without peeling-there are significant amounts of vitamin C in the peel) can be anything from a good to a very good source of vitamin C. The variation depends on which variety of apple you are eating. The vitamin C content about halves after three months storage-with the exception of 'Sturmer', a hard, acid, late variety that is rarely available today. Remarkably, it loses no vitamin C in three months of cool storage.

This information is from the 'Changes in Vitamin C Content and Acidity of Apples during Cool Storage', Askew & Kidson, reported in the NZ Journal of Science and Technology Vol. 28, No.5 1947, and from data from the New York experiment station reported in 1946.
 

Any variety which has from 6 to 15 grams of vitamin C and is not highly packed with sugars is regarded as a 'good' source.

Some very sweet fruit can be regarded as fairly good sources because they have more than 6 grams a serving, but not much more.

Any variety that gives from about 15 grams to about 30 grams can be considered a 'very good' source of vitamin C

When a variety has more than about 30 grams per serving, it is an 'excellent' source of Vitamin C.

* These values are for the flesh only, peeled of the skin. Apple skin, or the flesh just under it, has very good amounts of vitamin C -typically around 50mg/100gms, and for some cultivars, such as 'Granny Smith' and 'Sturmer', very good amounts, at around 100mg/100gms of peel. No one sits down to a big feed of apple peel, but it does mean if you eat the apple without peeling it you will take in more vitamin C than the figures for the flesh alone suggest.

Vitamin C content of apple cultivars  - no sources listed, but a page from the North American Fruit Explorers site listing vitamin C content of various apples.
http://www.nafex.org/jansonfiles/JansonJan68.htm

Please email any figures you have on the ascorbic acid content of apple cultivars-
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