Hmm, I am not sure if one mangaka made one manga that you like can be considered the favorite one. It’s more like your favorite manga than author I guess, but ok.
I think Tsugumi Ohba and Takeshi Obata would be my favorites. They made 2 of my favorite mangas: Death Note and Bakuman, which is also were adapted into one of the best animes (although they cut some stuff from Bakuman). But the their latest Platinum End is a bit disappointing.
Hiromu Arakama’s “Fullmetal Alchemist” is legendary. I didn’t read her other manga, but I watched Silver Spoon anime and it was quite funny, I think she is a good mangaka.
And CLAMP. Chobits and Code Geass. I didn’t read Chobits, but anime was great, so manga must be even better, I will check it out at some point definitely. Code Geass is an anime original which is very good. I am reading right now xxxHolic and Tsubasa Reservoir Chronicle which seem to be very interesting. So yea, CLAMP is my favorite definitely.