Best apples for apple pie

Some of this is a matter of opinion, as you must decide what type of apple pie you like. Do you want one with discernable applie slices, or one where the filling becomes somewhat like apple sauce?

Kenji Alt of Serious Eats has done an in depth review of 10 commonly available apples (at least in the US), and his conclusion is:

[...] the best ones in the running are the Golden Delicious and the Braeburn. They're the ones I use for all of my baking purposes.

Still, I suggest reading his review, and drawing your own conclusion.

He also shares that the level of acidity an apple has correlates with how long it takes to brown when cut and exposed to air--the slower the browning, the more acid. Those apples with the most acid tend to produce the firmest cooked product, as the acid helps keep the pectin from breaking down.

Granny Smith is the standard go-to. I think the best pies use a combination of apples. Alton Brown likes a combination of Granny Smith, Honeycrisp, Braeburn and Golden Delicious.

I love Granny Smith and Honeycrisp for out-of-hand eating, so I often have those on hand when I get a hankering for baking. A combination of those two apples do make for a great pie.

Having found this question on the Stack Overflow blog - I feel compelled to add a UK perspective.

Looking at Kenji Alt's list of apples, I find there are only probably three on there I've ever seen in the UK.
Here they are in what I would consider worst to best order...

• Golden Delicious - there was a massive influx of these cardboard/floury-flavoured apples in the 80s & 90s, now thankfully diminished. They don't taste of much raw, so cooked they're just going to be filler. They also turn to mush when cooked.

• Gala - a modern twist on an old English favourite. Great to eat raw, but they don't cook well. Too sweet & perhaps a little 'cidery' for pie. They remind me of Cox's orange pippin, but are apparently not closely related.

• Granny Smith - just enough punch to cook with, tart raw, but floury when cooked.

But there's one very serious candidate missing from that list & from all other answers here.

• Bramley [or to give it its full title Bramley's Seedling - this is the British cooking apple. Almost too tart & crisp to eat raw [though it has its adherents] & also comes in sizes big enough to make a pie from a single apple, this is absolutely the staple of the British apple pie.

Even Wikipedia calls it "the most important cooking apple in England and Wales"

Maybe US apple pie is aiming for a different flavour or texture profile than the UK - I've never had apple pie in the US - but if you can get hold of these [they are apparently available, if in limited supply, on the North American continent] then be sure to give them a try.

The secret is 3 different types of apples. Keeps every bite different & your taste buds will thank you.