My guess is if you asked BB, he'd say it's Paul Brown.
Pretty much, Belichick called him the Father of modern football. Belichick was tasked with defeating Brown's offense in the 80's with the Giants. Walsh was running it (and sometimes taking credit for it) but the WCO was Brown's invention when he started with the fledgling Bengals. The dearth of talent he was dealing with on the OL and at RB for the expansion Bengals had forced him to adapt. Walsh later refined it, of course. According to Belichick, it was essentially Paul Brown's offense right down to the same terminology.
If there was no Paul Brown, there would be no Bill Walsh and the WCO.