For me, lots of candidates, but I'll go with #589, Poohsticks Marathon, because of my history solving it. I worked on it for a month after I'd solved about 200

For me, lots of candidates, but I'll go with #589, Poohsticks Marathon, because of my history solving it. I worked on it for a month after I'd solved about 200 problems, and got nowhere. Dropped it for 2 years, then decided after I'd solved 500 problems that I should give it another go for solution 501. I'd learned plenty of techniques in the interim, and finished it off in a couple days. What's fascinating to me about this one, though, is its difficulty level, versus what the design team thought that level would be. Those levels are set automatically based on how quickly PE members manage to solve it, and this one took 32 weeks to hit 100 solvers, for a 95% difficulty. But problems are usually scheduled in batches of 6, one per week, with an expected difficulty cadence of Medium, Easy, Medium, Easy, Medium, Hard. And #589 was expected to be an _easy_ problem! I can actually see, having solved it, why the team thought it deserved its easy slot, but, wow, did things work out differently. For a more general favorite, I really enjoy solving any of the impartial game problems. I'm not pointing out which those are - part of the fun is discovering that that's what you're looking at.