from this three which one is the best best current measuring method?
Method A is the simplest. However without an opamp you will get low resolution and/or high voltage drop across the shunt. You can get around this limitation without any extra parts by using an MCU with built-in opamp (eg. ATtiny26L). This is how the rc-electronics Watts Up Meter does it.   

Method B (using a DC Hall effect sensor, not a current transformer) is good for measuring high current with low voltage drop. Hall effect sensors suffer from hysteresis, external magnetic fields, and high internal noise, so they are not as accurate as a shunt. Some Hall current sensors can read both positive and current currents, which is useful for measuring current going into the battery (eg. when charging it).      

Method C is just method A with an opamp and temperature compensation (which a good shunt doesn't need). 

If you are measuring high current then the shunt should have low resistance to minimize power dissipation, and 'Kelvin' connections to reduce errors due to parasitic resistances. In this photo of a Watts Up meter the Kelvin connections are the two thin tracks coming out below the middle of the 0.001&ohm; shunt.