Wild card
For a deserving challenge, answer, or user that isn't a good fit for any of the other categories.
quintopia's Befunge-93 in Befunge-93
Interpreting Befunge 93 is already not very easy, but doing it within Befunge 93 is a behemoth in itself. This uses the existing features of befunge to simulate the program within its own playing field!
Jo King's quantum quine
Nominated by emanresu A
When I wrote this question, I didn't believe it was possible to create a program with a score of 0. It's hard to make a program that functions when any character is removed, and even harder to make a quine that stays a quine when any character is removed, let alone one that behaves in such a specific way.
Jo King's answer uses Backhand, a language which skips every few characters to make restricted-source a bit easier, and even then it's still a quite complex answer.
m90's answer to Final tribute to John Conway: FRACTRAN self-interpreter
Nominated by Dingus
The previous best solution known for this problem, at 48 fractions, had stood since 2017. It was blown out of the water by @m90's 30-fraction solution, which was accompanied by an excellent explanation.
tjjfvi's answer to Complement a POSIX Extended Regular Expression
Nominated by Dingus
This challenge had gone without an answer for over 7 years, and a quick survey of the comments reveals just how difficult it was perceived to be: 'It's possible, but crazy tedious ... slightly harder than writing a complete RE engine!', 'Supporting escaped meta-characters is honestly going to be a challenge in itself', 'The code I have so far is basically 30KB'. And indeed, @tjjfvi's well-explained code is a complex beast, transforming regex → tree → DFA → regex, all in less than 1.6 kB! This answer could easily have been nominated for Most Complex Answer or Best Explanation, but @tjjfvi already has another worthy contender in those categories!
