Best rapper ever? well that's like the greatest guitarist or any other greatest ever argument made. It's going to be pretty subjective to personal opinion and preference. But, for sake of argument, discussion, and debate; I'll post my nomination anyway.
So, like many other people, I consider greatness to be something earned over time. So I'm not going to give this nomination to anyone who has came out recently, or even within the last 5 years. The Nomination for best rapper ever is....
Rakim,
AKA: God MC
Rakim, formerly of the duo Eric B. & Rakim, has been serving up flow lessons since back in the old school. He is a major influence to any and every rapper who ever amounted to anything. Rakim's lyical stylings along with his smooth and direct delivery make for some of the best verses ever put on tape. Rakim found solo success after splitting with Eric B., upon the release of 'The 18th Letter' (1997), and the follow up 'The Master' (1999). Rakim's latest release, 'The Seventh Seal' was released in 2009. 'The Seventh Seal' marks the second return of the great God MC, showing that he still has what it takes to run with the best.
Overall, I think there are a handful of rappers that can match the raw skill and delivery of Rakim, and also a few that can match his influence on the genre of hip-hop as a whole. But there is no one that has done both for as long as Rakim has.