Input impedence of Op-amp circuit
The first one is correct.

Zi = Ri

The input impedance is looking into the opamp with respect to ground. So any resistance between the input signal and ground, and since the inverting terminal is held at ground, the opamp will do everything it can to ensure that it is always held at ground.

This question has also been asked before, so its worthwhile to look through the site before posting. 

Input impedance of inverting amplifier clarification