The second one. With this layout you are protecting the microcontroller from damage, and also protecting the input signal from shorting to ground via the TVS when it is clamping a transient.

The second one.

With this layout you are protecting the microcontroller from damage, and also protecting the input signal from shorting to ground via the TVS when it is clamping a transient.

Edit:

@Puffafish is right. You will get the best protection putting the TVS at the signal input. So the first layout is better.

Anyway, putting a resistor before the TVS has the benefit of protecting the input singal (and the TVS itself).