This is one is genre specific, but Caesar 3. I love city builders and have played them for as long as they've existed. I've learned all the little tricks and systems of the ones I've played, exploiting esoteric mechanics and optimizing my little utopias and creating epic, sprawling empires that far exceed every metric asked of me. That said, Caesar 3 is a challenge I still relish after (oh wow, has it really been) 25 years. It's the only city builder where the "peaceful" branch in the story is harder than the "wartime" scenarios. I revisited it recently wondering if I was just missing something back when I was younger, but nope. On the harder levels it asks you to sustain larger and larger populations with increasingly limited resources, and reaching the level of getting patrician housing (only achieved with sustained, stable access to literally every amenity) is extremely difficult but oh so satisfying. Every other city builder I've played, I barely have to think about every house becoming the top tier, but in Caesar 3 it's impressive if even a single block achieves it. It stands out even now after so many new entrants into the genre. Hell, it's still worth playing haha.