What is the ideal value of input resistance?

depends on what you are measuring

practical use like electricity or batteries as low as possible

to test voltage on a car battery perhaps 0.1 ohm / volt

to check the final anode on a crt tv at least 1000 ohms per volt

low power circuits require higher impedance

wire connections low impedance

the 10000 ohm per volt meters are fine as fixed monitors but usless for practical service work. they indicate voltage thru oxides and lose screws

they show 120V in a socket when the lamp refuses to light