I'm not a fan of him personally, but Nelson Piquet's move on Ayrton Senna at Hungary in 1986 was unbelievably good. Great car control and fantastic pass on a guy who was, if I recall, pretty good at making his car very wide!
http://www.youtube.com/watch?v=Gkirs...eature=related
As far as the greatest overtaker is concerned I'd go for Nigel Mansell. Silverstone 1987 was good, as was his move on Senna at Hungary in 1989 and the pass around the outside of Gerhard Berger at Mexico in 1990 (on old tyres!).