Their obviously rivals but if all three went to all out war, pulled all the stops and just fought to the death who would win? My money is on Grazzt.
Orcus doesn't do much for me he seems a little stupid giving up godhood because he didnn't like the name Tenebrous well freakin change it then that was a stupid move... or better yet why not at least wait until after deposing your rivals to quit being a god. I'm sure he could have been way stronger then.
Demogorgon is cool, the pic of him in BoVD has got to be humiliating he looked purty gay to me. What did he try anfd horn in on Yeenoghu he looked downright Gnollish. And another thing in freakin Bastion of broken souls he's trying to kill himself. he just does not impress me.
Grazzt on the other hand is awesome his pic in the back of FC1 with Iggwilv was awesome looking (if not a bit drowish). plus he controls more layers than anyone else in the Abyss.
I give it to the new Prince of Demons Grazzt.