I have listened to many musicians and musical genres over the years and I believe that Eminem is the greatest rapper and musician overall. I would like my opponent to argue that another musician is better. I would rather not argue against a musician of classical music because it is very difficult to compare. Good luck challenger.
Personally I think Eminem isn't even the best rapper. There are a multitude of better rappers, and musicians. All in all, you can't really prove either side, as it's entirely opinion based. There are some who believe this, others, like myself, who thinks he's pretty bad. There can't be a "best musician of all time" as everyone has their own idea of the best musician of all time.
I agree that the sound of music is subjective and some people like different genres and artists over others. But i also believe that looking at lyrics, something concrete, is a good way to compare the skill of musicians outside of the instrumental genres.
But on what grounds are they compared? Again, it's too opinionated. Sure I think that some artists are much better lyrically than others. But some might like the whole three words per song type of song and might find that more skilled than actual thought out lyrics. But disregard those people as I'm sure most would agree that three words per song doesn't take much skill. How do you compare lyrics, how do you chose which one is better? If we put Eminem up against Dave Grohl, on what grounds would make one better than the other?