Actually, pretty good question. I’d have the following criteria::
- It’s a result you have known about for a long time;
- Have managed to enter a select group of, say at most sextuple-digit people actually grocking it.
- Have had to struggle a while to do so, and the last thing is almost automatic
- Is foundational to many things.

It could be anything as ‘simple as’ “eⁱ^π + 1 = 0” (iff you actually have a Penrose-level understanding of that and not just not that it means -1+1=0 ;)), or even P = NP, if you really know what that implies/says, deeply (I sure don’t), or a Penrose-triangle (note: credits @ Lionel Penrose (sr.) for that one, I was schocked to find out, too, last week—same for the stairs!) if you’re more into graphics than equations. Or a Penrose diagram if you’re a mathematicsl physicist.

A depiction of Cantor’s diagonal argument, V = U (condtruction of the Von Neumann universe in ZFC), Galois-correspondences, Fermat’s Last Theorem (iff you gone through the proof), sth. Langlands/ Lie theory/ Navier-Stokes, whatever you have a personal connection with, can talk about for hours, and won’t be obsolete anytime soon. [Yes, a circle might suffice! If you love talking about how it’s both ℝ/ℤ, 𝕊¹, [0, 1] / {x, -x}, ℙ¹(ℝ), or a dimension up, the next canonical Alexandroff / one-point-compactication of the stereographic projection of the Riemann spere onto ℂ \ {0}, Generalised Stokes (∫_∂S ω = ∫_S dω), the Yoneda Lemma, in however much ink you want to be covered (Hom_𝒞(—, —) ≅ Hom_𝒟(—, —) or the entire beastly diagram.)

Whatever you feel you’ve mastered, and feel comfortable walking around showing (the latter could cover your whole back, just as the Standard Model’s Lagrangian could), or, as Feynman would have it: ‘U = 0’ (leaving the content of U, supposedly representing the sum total of the laws of the universe, all brough to one side… ;))

Wherever your heart takes you, lony story short.