Between the two that you picked, Eminem is the clear winner. Much more dynamic and prolific than Diabolic. My pick would be Kendrick Lamar. I don't want to get into the whole mainstream/underground crap, because realistically just because underground rappers are edgy and don't follow "the system" doesn't make them anymore skillful. I would also put MF DOOM over Diabolic any day. I hate when people dash an artist because they're popular, it's such an annoying hipster attitude. One of Kendrick's biggest hits (Swimming Pools) is an emotional track about alcoholism that ties into the story of GKMC. He's a top notch lyricist, and he knows how people will react to his music, so he accomplishes his intentions. TPAB was one of the most skillfully articulated story albums I've ever heard, all while remaining socially conscious, relevant, and reviving the idea of a feel-good emcee (such as KRS-One) with tracks like "King Kunta" and "I". Also, I find it a tad pretentious that you only put the two options you did in the poll. There are many great artists out there that contend with those two on many levels.