Shor the Duck's SG Collection, a curated problemset split into difficulties of the best problems on Codebreaker set by the SG IO Community!
Criteria for 6 star:
Can require somewhat advanced techniques or data-structures, and usually forcing at least some thinking/observation
(Lazy propagation segment tree, Sqrt Decomp)
Should be challenging for Codeforces Experts and suitable for Codeforces CM
Above NOI 2024 q1 (problemsetter) level, including NOI 2024 q2 (shops)