When Immortal Technique came out a handful of years back, his main strength was his freestyle. Probably my favorite track by him is Obnoxious.
Like or not Em, probably the best of the last 15. Had the chance to see that weird white boy coming up. Was at the now infamous Scribblejam where he got taken down. This was just before he blew up.
Most of the heads heard mixtapes with him on it, but to him in person was ridiculous. Juice though pulled off something off the flow that was unimaginable to pwn him or any person on the earth on that day.
The only place Em lacks, is voice.
From my life, touchstones. Obvious ones, cause no matter how we try to protest, the best float to the top:
Boogie Down Productions
Early Ice Cube (Death Certificate remains in terms of production and shear career turn around one of the greatest albums out there)
Dre (for his "production" or at least production supervision)
Kool Keith (toooo much to say about this guy, even back in Eletromagnetic MCs the guy was light years ahead of the game in terms of rhyme structure especially abandoning end rhyme for middle rhyme and assonance. His later work is just mind blowing)
Biggie (his freestyle like Em was also just sick, game changer in terms of syncopation, completely off-beat rhyme at times)
Jay-Z (not a big fan of his stuff, but in terms of freestyle and his early flow, no one could touch the ease with which he could float over any beat. Stupid charisma live.)
Em. (Arguably the best body of work. Greatest breath control ever. From subject matter, play on words, complex rhyme structure freestyle or written, charisma, on and on and on. He's latest stuff is just silly, but a testament to the fact the guy know how to make millions even when mediocre.)
Tons dropped off. All the stuff mentioned above is good. But seriously, when it comes to stuff that heads can listen to and also crossover, these are the best.
All but Kool Keith got what they deserved.