You're right, I was wrong to say "most challenging" as there have been harder ones coming out recently. I think the correct statement would be "most challenging long-standing benchmark" as I don't believe any other test designed in 2019 has resisted progress for so long. FrontierMath is only a month old. And of course the real key feature of ARC is that it is easy for humans. FrontierMath is (intentionally) not.
