Purim is a very great day. It is greater than Shavuos, because they were forced to accept the Torah on Shavuos. Har Sinai was raised above their heads, thus requiring them and forcing them to receive the Torah. On Purim, the Jewish nation accepted the Torah from love (see Shabbos 88.). So in that aspect Purim is greater than Shavuos.
Purim is also greater than Pesach, because Pesach we celebrate liberation from slavery to freedom, while on Purim we celebrate salvation from death to life. Thus, Purim is greater and holier than Pesach and Shavuos. ~ The Chasam Sofer zt’l (Drushim p.156:) as quoted by R’ Elimelech Biderman Shlita