The most difficult math problem ever solved, according to the Guinness World Records, is Fermat's Last Theorem, since it took over 300 years to solve and has precipitated more incorrect proofs than any other math problem. Mathematicians are quite interested in a problem called the Riemann hypothesis. There is a one-million dollar prize for its solution, and very little progress has been made in solving it. I don't think that just one is the most complex 'ever'. But if your looking for extremely complex equations, your best bet is unsolved ones, because most offer around $1,000,000 for a correct proof.