I was going to say, I thought their self-titled was their debut. It was a damn powerful album, and few (if any) weak spots. I thought about that one as well as a few others some have posted. My "honorables" were just more than anything some of my personal favorites. I don't think any rap album should even be considered in the discussion, although I love rap. I don't think a lot of others listed should be considered in any "greatest" category.
My criteria were twofold: Critical success and mainstream (sales) success. "Groundbreaking" could be a third - something that changed the whole entire scene of music ('Nevermind' wasn't Nirvana's debut, but that album totally destroyed "hair metal" and similar styles; some of the punk stuff here [Ramones, etc.] deserve mention) I don't think much more can be assessed, IMO.