Tongue-no-longer-in-cheek:
I reckon the C core of Emacs is some of the most battle-hardened code out there. Verification, a-la SEL4, is probably irrelevant but still nice. Guile is modern and performant but Elisp is still its own little joy. Literate programming is always nice until it gets in the way. Straight is good enough for me now. Macros are always cool and a leaderboard would be fun, but patch algebra is really nice, see jujutsu nowadays. And beard length is gendered and so only partially admissable. Infinity categories are way out there and always good for a reference.