What is the Greatest Sin with Allah?
It is the major Shirk (polytheism).
Evidence from the Noble Qur'an
{Verily whoever sets partners with Allaah, then Allaah has forbidden paradise for him and the Fire will be his abode}, {Soorah al-Maa.idah, verse 72]
Evidence from the authentic Sunnah
((The Prophet was asked : what is the greater sin , he said: To ascribe partners to Allaah even though he created you)), [Transmitted by al-Bukhaaree]