What's the difference between FET ESC And NFET ESC ?
And which ESC best for multi rotor.

ESC's with SimonK firmware are best for mutlirotor.

What about that can u tell me
http://www.quadkopters.com/product/esc-ubec/simonk-esc-30a/

Nfet only is better than mixed fets for most Multirotors.  Use esc with nfet with Simonk. Fets are responsible for the pulse ur motor recieves.

Can u suggest me ESC or which site I buy it.

Normally N Channel FET have low resistance than P channel FET, Lower resistance help to reduce heat and increase efficiency. Don't know any other factors involved in case of ESCs.

N= negative , P= Positive,   FET= Field Effect Transistor, In   related electronic circuitry,  similarly we have Traics, MOSFET= Metal Oxide Semiconductor Feild Effect Transistor,  a type of Transistor,  so on...FET are also used in most drive circuitry like Audio output circuits, ESC Controls,  Lightings drive, Power Supply,  SMPS power etc,  N= negative,  P= Positive depends on designer circuitry,  for performance,  and requirements,  FET N or P depends on design for performance and needs...

i would suggest buying the simonk esc 30A from quadcopters.com or rcmumbai.com. Also rcproducts.in has the AFROESC which i really like.

You can check it out at rcmumbai.com or quadkopters.com. There are several other LHS too but since your are near to Mumbai these are the best LHS for you.