The immortal A5 chip
We see the A5X come and gone in less than a year.
The A6 now only lives in one device, the iPhone 5c.
A6X also come and gone.
The A5? It's in the iPhone 4S, 5th gen iPod Touch, Apple TV, the (also immortal) iPad 2, and iPad mini. All are still sold by Apple.
It's the hardware version of Windows XP from Apple. :D