Both are Naoki Urasawa at his best. Monster is 18 volumes: a surgeon hunts the patient he saved, who grew up to be a killer. Pluto is 8 volumes, a retelling of Osamu Tezuka's Astro Boy arc "The Greatest Robot on Earth".
I say Monster first for the slow burn, then Pluto, which lands harder once you trust Urasawa's pacing.