For me, in the late 90's Duncan. It depends how you mean by head the ball, ie how accurately they headed it or how well they attacked it. Ferguson for a period under Royle and Kendall was a terrifying prospect and attacked the ball aggressively. Some of the headers he scored, against United & Liverpool at home were as good as you'll ever see. There was also one away at Arsenal used for MOTD and you see the corner come in, it goes over a jumping Adams, Bould, Seaman and out of the blue you see Ferguson meet it about 2 foot above them and head it in. Astonishing really.
From my time watching Everton I'd also say Gary Speed deserves to be on that list. Not the best but he was also very good in the air and is worthy of a mention.