nickel-62

Nickel-62 is a stable isotope of nickel, having 28 protons and 34 neutrons.

It has the highest binding energy per nucleon of any known nuclide (8.7945 MeV). It is often stated that is the "most stable nucleus", but this is because 56Fe has the lowest mass per nucleon, not binding energy per nucleon, of all nuclides. The lower mass per nucleon of 56Fe is possible because 56Fe has 26/56 ≈ 46.43% protons, while 62Ni has only 28/62 ≈ 45.16% protons. Protons are less massive than neutrons, meaning that the larger fraction of protons in 56Fe lowers its mean mass per nucleon without changing its binding energy, which is by definition measured with respect to the actual mix of protons and neutrons in the nucleus (even though free neutrons are unstable.) In other words, nickel-62 can be said to have the 'least massive' protons and neutrons of any isotope.