On a technical ability Eminem is one of the best ever - even when he's doing songs where he's doing a gay persian voice the whole time, the stuff he does with rhyme schemes is insane - but that doesn't make anything after MMLP any less unlistenable. It's funny that one of the earliest beefs was with Canibus, who was world champion 'guy who can rap his ass off without making very many good songs' til Em came along.
Admittedly a large part of my disgust/animosity for Em comes from his fanbase. I'd be less annoyed by him coming up in discussions of 'best ever' if the people bringing him up didn't constantly admit they only listen to him, Immortal Technique, and Tech nine or something equally cornball. I think after I heard a trust fund kid say "I think Eminem is the best rapper ever, and that's not just because he's white" I became instantly suspect of anyone who's go-to favorite rapper after 1999 is eminem.