Question

should the resistance of an ammeter be low or high

Answer 1

The resistance of an ammeter should always be negligible or zero so that the current passing through the circuit does not get opposed and no error occurs in correct reading of the current.

Answer 2

 Ammeter resistance should be very low. Ammeter is connected in series in a circuit.So if resistance is more, current won't flow through the ammeter.

A device which measures current needs a low resistance(close to zero) so that at least somewhat of the original current flows through it and the current can be measured close to original value. 

If the ammeter had high resistance then, the current flowing through it will be certainly lower than that of the original current which is present in the wire and hence leads to the wrong measurement of current.