Biggie and 2Pac, oh man, they both had such charismatic personalities that came through in their raps; same with the usual West Coast suspects (Dre, Snoop, Cube) and the members of the Wu-Tang, especially Meth, Ghost, and Raekwon - honestly, I've been on a huge hip hop kick for the last few months, I guess I just kinda got burned out on most metal (but I still love writing it, and still listen to it about 30-40% of the time), and it really is their immediately identifiable and powerful styles that I find so captivating, as well as the stories they told and the sheer skill of their rhymes; I'm a great appreciator of awesomely phrased guitar solos, and I appreciate the awesome flows of the aforementioned MC's in a similar way.
Some highlights: BIG's fucking amazing duet with Method Man, tough to think of a better rap duet honestly
2Pac IMO had the most powerful delivery simply as far as the force he put behind his voice; it's all from the diaphragm, and goddamn what a groove (starts at 1:00)
I love Wu-Tang for how creepy and evil their sounds was (thanks largely to RZA's productions); this one is so metal