During the process of fertilisation the pollen tube of the pollen grain usually enters through which end into the embryo sac ?

Through micropylar end.

Micro Pyle is located at the point of joining of both the pollen tube and the ovule. The pollen, the male gamete enters into the pollen tube follows the three routes to reach the ovule for the purpose of fertilization. These three points are porogamy, mesogamy and chalazogamy.
(1) Porogamy:
The pollen grains enter the ovule through the microPyle opening in the ovule.
(2) Mesogamy:
In this type, the pollen grains enter through the funiculus or the integuments.
(3) Chalazogamy:
In chalazogamy, the male gamete pollen enters the ovule through the chalasal tissues.
In all the above routes, the pollen grains enter the ovule. But these routes often reach the micro Pyle as the final destination. Hence a pollen grain enters the embryo sac through the micropylar end.