What is the best shape to design WiMAX microstrip patch antenna?

PIFA is a good choice

TQ. actually i'm new in this field. may i know what is PIFA? And why it's a good choice?

It is Planar Inverted F(shape) Antenna...........:)

If u intend only for Wimax then use PIFA or slot Antenna

cross slot square patch microstrip with diagonal probe feeding for dual band 802.11a/b

PIFA is some what old shape.......u can go to modified U-shape or triangular shape for generating the WiMax frequencies.....