Honestly, most people instantly yell 'Sherlock Holmes' and they're not wrong, but it feels a bit lazy to just list those. His Holmes stories are obviously the bedrock—'A Study in Scarlet', 'The Sign of the Four', those are where it all began, and they're essential. But for a single book that really captures the character, 'The Hound of the Baskervilles' is the peak. The atmosphere on the moor is just unmatched.
He wrote a ton beyond Holmes though, even if it's mostly forgotten now. I tried reading 'The Lost World' once, the Professor Challenger one. It's a fun adventure romp, dinosaurs and all that, but the prose feels a bit clunky compared to the crispness of the Holmes tales. I think his popularity is almost entirely tied to that one detective, which is a bit of a shame for his other work, but it's also a testament to how perfect Holmes was.