ARM. Every second that passes, for each person that is born that will use a computer, at least two people who will never use a computer is born. Those two people will own a phone though, and that phone will have an ARM chip.

As time goes on, more and more ARM chips will take roles traditionally taken by x86-64. Hell, ARM is already the best-selling architecture. Laws of scale will dictate that investments in x86-64 will fail to keep pace with ARM. Apple Silicon is already showing a small fragment of that effect. The chips are incredibly competitive, and for what Apple has chosen to focus on (perf-per-watt), unbeatable. ARM investments by other companies are catching up to Apple, and x86-64 does not make enough money to reverse that trend.

There's a small hitch. Unless China is allowed to directly pilfer ARM IP, they're going to pull off a RISC-V (or homegrown ISA) transition at the drop of a hat. And if American phone manufacturers are intent on diversifying, they very well might too; there's little forcing you to use ARM as your RISC ISA on Android or iOS. Competitive ARM cores don't grow on trees, and the licensing fee does't make it very attractive to compete unless you have a de-facto ISA license like Apple or Nvidia. And both of those companies have dabbled in RISC-V too. The intent to usurp ARM's throne has been clear ever since Nvidia was blocked from outright buying it in it's entirety.

Look at it this way; Apple can design their own cores whether they use ARM or RISC-V, and control the software from top to bottom either way. Nvidia's already shipping RISC-V microcontrollers to cut down on manufacturing margins, and without better options the rest of the world might follow. ARM's dominance is only possible if better RISC options don't exist; and for anyone that's not ARM the idea of IP serfdom sounds awful.