This poll's is waaaay back 2008 taken by Oricon, but then I felt like posting this since some of my biased mangaka are in the list.
PS: Sorry for the crappy image. Some are really hard to find.
1. Yazawa Ai -- NANA, Paradise Kiss
2. Toriyama Akira -- Dragon Ball Series, Dr. Slump
3. Inoue Takehiko -- Slam Dunk, Buzzer Beater
4. Adachi Mitsuru -- Touch, Cross Game
5.5 Togashi Yoshihiro -- Hunter X Hunter, Yu Yu Hakusho
5.5 Oda Eiichiro -- One Piece
7. Urasawa Naoki -- 20th Century Boys, Yawara
8.5 CLAMP -- Tsubasa RESERVoir CHRoNiCLE, CardCaptor Sakura
8.5 Fujiko F. Fujie -- Doraemon
10. Tezuka Osamu --- Astroboy, Black Jack