The judge tier list, explained

This is the opinionated version of where every judge, book, and resource sits, and why. For the interactive tier list you can sort and filter, see Judges and Online Resources. Here I actually explain the placements.

The ranking measures one thing: how much getting good at this makes you good at solving unseen problems. Clean problems, honest difficulty, and a culture that rewards reasoning go up. Memorized sets, grindy sheets, and solution video culture go down. Popularity is not a factor. Codeforces is not at the top just because it is huge.

Yes, WhiteBox is on the list too, at number one. We are biased, and also right. Everything below is the outside world.

S tier: train here

• AtCoder. The cleanest problems in competitive programming, beginner to red. The Beginner Contests are the friendliest starting point anywhere, so if you are newer this is where you go, but the ceiling is just as high as the floor is low: the Regular and Grand Contests, ARC and AGC, hold some of the hardest and most original problems in the world, the kind the strongest competitors specifically train on.
• CSES Problem Set. 300 classic problems covering every core topic in order. Free, no noise, the best systematic coverage of the standard toolkit.
• USACO Guide. Free, topic ordered, Bronze to Platinum, with problems and editorials. Structured the way a roadmap should be.
• MarisaOJ. Underrated. Problems organized by algorithm and difficulty for targeted drilling on exactly the thing you are weak at.
• QOJ. Where ICPC, Universal Cup, and other high level contests live. Hard, and the real deal once you are past the basics.
• YouKnowWho Academy. The most comprehensive topic list anywhere, by Shahjalal Shohag. Aimed at more advanced users, but the coverage is unmatched.

A tier: great, with a caveat

• Codeforces. The largest source of fresh, original problems and live contests, and ratings are the standard skill measure. It is A and not S because of the culture, not the problems. The feed rewards the wrong things. Train on the problems, ignore the noise.
• CSES Handbook. Antti Laaksonen's free Competitive Programmer's Handbook. The best intro book there is. Read it first.
• CP-Algorithms. Reference implementations and explanations for nearly every algorithm you will need. Bookmark it.
• Aizu, Luogu, and Baekjoon. Massive Japanese, Chinese, and Korean archives with structured courses, huge catalogs, and difficulty tiers. Enormous depth if you are willing to dig.
• oj.uz, Universal Cup, and Library Checker. Olympiad problems with clean statements, the team rounds that essentially every top competitive programmer in the world takes part in (Universal Cup), and a place to verify your template implementations against reference problems. Specialized, and excellent at what they do.

B tier: good for what they are

• Kattis. Clean ICPC style archive used by many regionals.
• OI Wiki. Comprehensive Chinese language algorithm encyclopedia. Deep reference.
• DMOJ. Open source modern judge, broad language support, clean UI.
• ZeroJudge. Beginner heavy set, popular in Taiwan.
• getcracked.io. Paid interview and competitive practice. Fine, but you can get most of it free above.

C tier: fine, dated, or niche

SPOJ, UVa, Timus, LightOJ, Yandex Contest, eolymp, TLX, and CodeChef. Classic archives and regional platforms. Plenty of good problems buried in here, but dated interfaces, uneven quality, or a narrow focus keep them out of the top tiers. Reach for them when you want a specific classic, not as your main judge.

D tier: weak for real practice

• LeetCode. Fine for rehearsing the interview format right before an interview. Weak as a training ground: the easy and medium set is heavily memorized, so your number there does not measure what you think it does. Do not live here.
• Codewars. Gamified small puzzles. Light entertainment, not training.

F tier: skip

• NeetCode. A repackaged LeetCode roadmap. Interview focused, dated, and not real CP. The video format actively trains the wrong habit. See is NeetCode actually bad.
• Striver A2Z DSA Sheet. Huge and comprehensive, and that is the problem. Enormously grindy, and you will get more out of any judge above for the same hours.
• HackerRank. Basic skill drills. Not useful once you are past the absolute beginning.

None of the F tier is evil. It is just the worst use of your time when better options are free and sitting right above them.