There's a lot of fun problems on Brilliant, but a select number that I have seen give you that really nice feeling to get right. It's hard to describe well, but some problems aren't too frustrating/easy, not too obscure-theorem-based, but thought candy that uses simple riddles to lead you to the answer, whether it be Algebra, Geometry, or anything.
I really enjoy these problems. If you have a favourite, please share them. I know I'm being vague, and I wish I had an example, but if you wrote anything you are particularly proud of for being easy but hard, hard but easy, I'd very much like to enjoy this type of maths again. Let me know if you know what I mean, pardon my comma splices.