Music overall is subjective, but the technical aspects are less so. Just as a discussion on the best singers can be quite technical, so is the discussion of the best rappers from a technical perspective.
No one is questioning Eminem's technical rapping; is flow is amazing and he has dope lines. The problem I have here is that the ONLY way to argue Eminem is the best rapper of all time is if you simply ignore or don't know anything about rappers who perfected the multi-syllable rhyming pattern.
Eminem's entire first album is almost a blatant take on Nas' style.
http://www.youtube.com/watch?v=X4qYox-Gb70
That's not to say Eminem "ripped off" Nas. After all, he didn't create the multi-syllable pattern, he re-did it for 90s rappers. Consider Kool G Rap who created that style in the 80s, and perfected it into the 90s
http://www.youtube.com/watch?v=UNGIahjziiE
Jay-Z was once quite a technical rapper
http://www.youtube.com/watch?v=cuaGcm8yqzI
Here's the problem for Eminem: all the rappers mentioned have better albums than him. Technically they are better. You can argue on lyrics except for Nas, who is clearly better - and even Eminem wouldn't deny that. For all this bullshit about Eminem being new rapper's favorite rapper, NAS is actually your favorite rapper's favorite rapper nine times out of ten.
In terms of new rap: overall the multi-syllable style isn't used much. Rick Ross does it but most mainstream rappers don't, and it's mixed in the underground. Some modern examples
Elzhi (also from Detroit, and co-signed by Eminem)
http://www.youtube.com/watch?v=7OI8Y8X4YkE
Reks
http://www.youtube.com/watch?v=oF7AU12LMB0
Roc Marciano
http://www.youtube.com/watch?v=m4Ec0HAgUSs
I'm not saying those rappers are better than Eminem mind you, just they have a similar rap pattern which unfortunately isn't used a whole lot today.
No one is taking anything away from Eminem. I'm merely arguing what he has argued before himself: he's not the best, nor is his style new or inventive. He doesn't match up with the greats in part because his albums don't.