O’Neill was (as a kid) and is (as an adult) one of my favorite comic artists. His work on Marshall Law was absolutely brilliant and all of his work on the League Of Extraordinary Gentlemen puts him into the comic legend category. Marshall Law got me through some tough times as a teenager (as well as Marvel’s Epic magazine an ROM if you can believe that) so I have a soft spot for everything O’Neill. If you haven’t read the League comic, I cannot recommend those enough– just make sure to start with Volume 1, then 2, then the Black Dossier and move on from there to Century: don’t skip around or start with River of Ghosts or something like that.
Here is a random sample of his work (trying to go chronologically at least).