I am new to Bicycling. I have a question on gear shifters. Mostly the bikes are available with Revo Shifter and EF Fire Shifters. Which one of these are best and why it is?
There is no best. It is purely preference.
Most high-end bicycles are equipped with the rapid fire levers - but this may be purely down to market consensus ie. what sells a bike.
Some XC racers still use the gripshift shifters and SRAM still make the high-end shifters.
I used gripshift years ago on my first mountain bikes but found them quite fragile in a crash.
Gripshift - you can change a whole stack of gears in a single twist. Rapid fire requires multiple few clicks to achieve the same movement. But this has not been a problem for myself in practice - if you read the terrain ahead.
The rapid fire levers are more robust and I am yet to break one.