I remember reading a thread about why Jay-Z gets so much props yet the person who started the topic, just couldn't get why it was so. After a lot of name calling going on in that thread, I gave my opinion that, I guess you either get Jay-Z's music or you don't. I'd say the same goes with 2pac's music.
When I listen to the song "me against the world", I'm always loving 2pac's delivery on that song. I know how the song goes but, whenever I try to ad lib to what he is saying in some parts, I just can't. I think it's only one small bit on the final verse and a bit of the first verse that I can rap along to. Then there are songs like "unconditional love", where he simply just raps over a beat and, I pick up each and every lyric on the song. True, its a simply written sing but its powerful to me. All in all, whatever style he decides to use, the lyrical content is amazing. That's why I can listen to a 1995 record of 2pac a lot more than any other hip hop music that is put out today.
Eminem, to me is the current definition of emcee and studio rapper. I hear him freestyle over a beat and, I think its sounds awesome. I hear him rap on his album and, I think it sounds awesome (relapse LP disclaimer). I think that's why as dodgy as some of the topics on his songs, he comes in 2nd for me, after 2pac.
Anyway, when people say 2pac's lyrics sucked, I have NO idea what they mean. They may not be complex or technically put together the way other rappers would but, the content is what rules.