Good rap is about three things: lyric, rhythm and rhyme. You can't just string words together without it meaning something. Rap's descended from griot so it should be telling a story, whether it's some deep social exploration or just the relief of hyping yourself up when you spend most of your time constantly making yourself small and non-threatening to survive. You get this full range of possibilities, anger and confidence and vulnerability, you get conflict resolution without violence, and because the focus is on the craft you get absolutely some of the best writing of any genre of music, and it's catchy as hell. It's not all Kanye and Jay-Z (though they're both really talented), one of the greatest rap albums ever is still The Miseducation of Lauryn Hill. Faithless is usually considered more dance but they've got some amazing lyrics from when Maxi Jazz was the frontman and of the more traditional stuff I think Mos Def and Chance are both pretty accessible. Maybe Kendrick too, but he does have more of the swearing and slurs that turn a lot of people off.