You would put Cowens above Horford

?
How kind of you. Horford couldn't tie Dave's shoelaces - not even close.
Horford is a very nice PF with a solid all-around game (except for rebounding).
Cowens was league MVP in '73 and was the best or second best center in NBA during his prime.
Either people forget or were not around to see that 70's team - they remain my favorite Celtic title group. And you are correct that Paul Silas was hugely important. Him and Cowens pounded teams into dust on the boards - God, that team was fun to watch.
Stating that Silas was THE inside guy for those teams is incorrect. He was a great rebounder and defender, but Cowens ruled the post. Cowens had a very good back-to-the-basket offensive game and was a better rebounder than Silas, though both were great. I always remember Cowens' ability to grab a rebound while turning upcourt in the air in order to direct his outlet pass. You don't see that ability much these days.