I have no other way to answer this question, so I am going to go by statistics (Billboards):
FEMALE
Jennifer Lopez
01. If You Had My Love (1999)
01. I'm Real (2001)
01. Ain't It Funny (2002)
01. All I Have (2003)
03. Love Don't Cost A Thing (2001)
03. Jenny From The Block (2002)
08. Waiting For Tonight (1999)
10. I'm Gonna Be Alright (2002)
12. Get Right (2005)
18. Play (2001)
32. I'm Glad (2003)
64. Hold You Down (2005)
72. Baby I Love U (2004)
Beyonce
01. Crazy In Love (2003)
01. Baby Boy (2003)
01. Check On It (2006)
03. Naughty Girl (2004)
04. Me, Myself, And I (2004)
57. Dangerously In Love (2004)
Ciara
01. Goodies (2004)
02. 1, 2, Step (2005)
02. Oh (2005)
96. And I (2005)
Christina Milian
05. Dip It Low (2004)
27. AM To PM (2001)
100. Whatever You Want (2004)
Amerie
08. 1 Thing (2005)
23. Why Don't We Fall In Love (2002)
51. Talkin' To Me (2002)
MALE
Usher
01. Nice And Slow (1998)
01. U Remind Me (2001)
01. U Got It Bad (2001)
01. Yeah! (2004)
01. Burn (2004)
01. Confessions Part II (2004)
02. You Make Me Wanna... (1997)
02. My Way (1998)
03. U Don't Have To Call (2002)
05. My Boo (2004)
08. Caught Up (2005)
58. Think Of You (1995)
59. Can You Get Wit It (1994)
60. Pop Ya Collar (2000)
Mario
01. Let Me Love You (2005)
52. How Could You (2005)
74. Braid My Hair (2002)
Chris Brown
01. Run It! (2005)
07. Yo (Excuse Me Miss) (2006)
Ne-Yo
06. So Sick (2006)
36. Stay (2005)
Omarion
27. O (2005)
94. Touch (2005)
For the females, I would hardly call Jennifer Lopez R&B, but since she has been in the industry the longest, she obvious has enjoyed the most success, however, it seems that anything Beyonce touches turn to gold, as all her singles have landed in the Top 5, I believe she is the strongest to reckon with currently. For the males, without a doubt, with both longetivity and overwhelming success, Usher is the original with the most success. However, my favorites are Christina Milian and Ne-Yo!!! <333 "Dip It Low" and "So Sick"

I hope this data analysis proves helpful to you guys!!!
