I would say Roller Coaster Tycoon is his best design. It was just before 3D became forced mandatory for games for some reason and not a one man job any more. I mean at the same time Sim City was actually good too.
This often meant a regression in graphics quality (e.g. Baldur's Gate 2 vs. Neverwinter Nights) and frequently a regression in gameplay too (usually due to horrible controls and/or camera, something like Monkey Island 4 is a good example).