It irritates me when people classify "mainstream" as a genre, when it's just a term to categorize what's hot and what's not. I enjoy a tune that makes me wanna dance, no matter what the lyrical content, and as long as the singing/speaking/rapping is legible (i'm talking to you, Ke$ha). I also happen to listen to punk rock and other subgenres from the likes of Paramore, Linkin Park (their recent material has been sucking though), and etc. If it's pure rap and hip hop music you're talking about, take it from the veterans of the last decade (Eminem, Jay-Z, Lupe Fiasco, Kanye West), or take it old school (like Will Smith, lol).
Now, the real problem for me lies in the new hip-hop of today (as well as those amateur teen sensations, like Bieber and Black). Black and Yellow? Wiz Khalifa? Waka Flocka Flame- motha fucking lame.