Source is just a magazine. But not an end-all be-all authority. I agree with much of the list, but disagree with a good bit of it.
50.50 Cent
49.Lupe Fiasco
48.Royce Da 5'9
47.Styles P
46.Rick Ross
45.Lil Kim
44.Canibus
43.DMX
42.Ludacris
41.Fabolous
40.T.I.
39.Prodigy
38.Posdnous(De La Soul)
37.Busta Rhymes
36.Lil Wayne
35.Bun B
34.Black Thought
33.Method Man
32.GZA
31.Kanye West
30.Guru
29.Snoop Dogg
28.Talib Kweli
27.Pharoahe Monch
26.Big L
25.Queen Latifah
24.Jadakiss
23.Mos Def
22.Common
21.Redman
20.Q-Tip
19.Big Pun
18.Kool G. Rap
17.The D.O.C.
16.Scarface
15.Slick Rick
14.Ice Cube
13.Andre 3000
12.Chuck D
11.LL Cool J
10.Melle Mel
9.Lauryn Hill
8.Big Daddy Kane
7.Krs-One
6.Eminem
5.2Pac
4.Jay-Z
3.Biggie
2.Nas
1.Rakim
My opinion
Should be higher
44.Canibus (sorry, Stans)
43.DMX
42.Ludacris
41.Fabolous
40.T.I.
37.Busta Rhymes
33.Method Man
32.GZA
19.Big Pun
Should be lower
46. Rick Ross
31.Kanye West
17.The D.O.C.
15.Slick Rick
10.Melle Mel
9.Lauryn Hill (as a musician shes in the top 10 easy but not as a rapper)
8.Big Daddy Kane