JH Conway. He has published work in a diverse set of interesting fields. I first met his name when looking for cool computer programs to write as a kid (ie. the Game of Life) but since then his name kept appearing in mathematics that I found interesting, whether it's Monstrous Moonshine or the properties of finite state automata. He has this incredible knack for turning anything he touches into fun - whether it's knot theory, group theory, quadratic forms, or, more obviously, combinatorial games. As well as working at the frontiers of mathematics he's discovered accessible but surprising and beautiful recreational mathematics, like Conway's soldiers. All in all, an amazing guy. Once of my regrets in life is being too lazy to attend his lectures on finite simple groups when I was an undergraduate.