Too many to list, hehe. But I could give you a list of my favorite well-known mangakas. I'll put my favorite book of theirs as well next to them.
Toko Kawai (actually I love all of hers equally, but Our Everlasting has a special place in my heart)
Masara Minase (Lies & Kisses)
Makoto Tateno (Yellow, Hero Heel)
Ayano Yamane (A Foreign Affair)
Takashi Kanzaki (Love & Alpha)
Yugi Yamada (Picnic, Spring Fever)
Hinako Takanaga (A Devil's Secret, You Will Fall in Love)
Kano Miyamoto (Two of Hearts)
Shiuko Kano (Affair)
Row Takakura (Skyscrapers of Oz)
Yuki Shimizu (Love Mode)
Also, Totally Captivated, a Korean yaoi by Hajin Yoo, is really good.
Hope this helps
