Okane Gai Nai is one of my favs (No Money in english) and Papa to Kiss in the Dark and Tsukiyono Fushigi is good as well. Haven't read many but they have good plot. Junjou Romantica is Great also....
Some mangas that i enjoyed are Bleach, Kyo Kara Maoh!, Kekkaishi, Katekyo Hitman Reborn, Death Note, Full Metal Alchemist, Skip Beat, Code Geass, Elfen Lied, and Eyeshield 21.
Well, it depends on your taste. Some people like well-developed characters and plot, and some readers just go for the sex scenes. Some well-liked titles: Junjou RomanticaGravitationZetsuai...