Landry, then Johnson, then Switzer. The people who responded Johnson are clueless. Johnson was the better general manager, but as a coach, he was good, but not exceptional. Landry was an innovator on both sides of the ball and sustained success over two decades. He invented the Flex defense, which popularized the 43 defensive alignment, and the shotgun. He also innovated in the front office, making the Cowboys the first team to heavily use computers to aid the scouting process.
Johnson was an even better front office guy. He brought a huge storehouse of knowledge of college football with him, and exploited it in his first few drafts, in which he had ridiculous numbers of picks after fleecing the dimwitted Minnesota Vikings GM in the Walker trade. Johnson innovated the idea of the trade value chart, he showed the value of being willing and eager to trade and move around the draft board, something which was uncommon beforehand.
Switzer was garbage.