I agree with what has been said with one exception.
The "best" apples MAY be local varieties that are available for a very short season and only locally. Of the varieties commonly available in supermarkets, I use Granny Smith for pies. This is important not only because many people do not have access to what I find the more interesting vaieties to use for pies, but also because Granny Smiths are available around the calendar; the others are not.
That said, I prefer using, when available, Northern Spies, Winesap, Stayman-Winsap, Ida Reds, and some of the local "heirloom" type apples that vary by area and may be available only at farmers markets or roadside stands .
Right now I'd use any apple I could get that tasted decent. I can't get a respectable apple from any supplier! When you saute' them with butter and carmelize with sugar, their isn't one drop of juice in the pan, the apples are sooooo dry right now, their like a sponge. Then they reach this point like a burst and turn into apple sauce.
If the apples were good, I'd use a combo of granny smith and golden delicious.
That's exactly what I do....mix varieties....granny smith and golden delicious make a great pie/strudel. I've got local honey crisps that are working well, Ark Black are too crisp and not that sweet....
As everybody knows, the VERY BEST pie apple in the world is the famous Belfiore di Ronzone
However, in case you have problems with the supply, my other favourite one is the apple we call Renetta. Don't know the American name but you can surely identify it as it's the apple commonly used in Austria to make Apfelstrudel...it's quite small, brownish/yellowish/greenish, has an irregular shape and a very sour taste.
Granny Smith is my favourite apple for salads as it isn't too sweet but not too sour as well.