There's a couple of programmers who's work impressed the hell out of me back in the day - these days it's difficult to single out 1 developer, as your never sure exactly how much people like Peter Molyneux actually do besides making demands.
David Braben:
He made Elite - and while I'm not a huge fan it was a massive achievement to fit so much into those little 8-bits. Frontier was impressive for the 16-bits as well. What impresses me most is that you can load these games on modern PC's via emulation, pump the processor speed, and the games just cope - they simply get smooth as silk. That dynamic approach was rare in those days.
Anthony Crowther:
He made some C64 games, but then he made Captive on the ST, PC, and Amiga, and it's my favorite game because of all the effort he put in - he made Captive massive, pretty much on his own. It won RPG of the year back in 91 IIRC; there's no way a 1 man show will ever win an accolade like that again. In todays terms, it would be as if a bedroom coder had come up with Oblivion on his own.
Roy Carnell:
He actually set me on the path to making my own games, he's written books on writing adventure games, using techniques he learned playing D&D at university. It's rumored that his Dark Crystal game was the first ever to have RPG style combat, this was in an article in one of those garish speccy magazines years ago.
But the main thing is that he's an incredibly nice bloke, he gave me probably £200 worth of books and games for the Speccy.