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.
These are much more recent games; things have bounced back and there's loads of 2D games today, but starting around the year ~2000 there was a decade or so everything had to be 3D, which often meant "3D FPS shooter", whether it made sense or not.
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).