MOV max clamp voltage
Protection of signal lines for ESD is better handled by components with much faster turn-on times than MOVs. You should be looking instead at components more like tranzorbs.

ESD voltage spikes have extremely fast rise times and one of your best strategies is to use a multi-forked approach.

First off consider carefully the types of interfaces that you use for your off board interconnections. Connector types, shielding and signalling impedance are very important considerations. You need to be aware that some interface protocols are just not going to be suitable to the types of things needed to harden a product against ESD. 

Next off include some capacitance on I/O lines to absorb the initial energy of ESD pulse. This changes rise time of the pulse (in conjunction with any signal wire impedance or series resistance you may choose to add and the capacitance value of the capacitor to ground). The best way to increase the signal line impedance coming from outside your board is to pass the signal through a ferrite bead type component. These will increase impedance significantly with increased frequency of the disturbance signalling. The fast rise time of ESD pulses has significant high frequency content.

The tranzorb you select will never be as close to the 5V cutoff level on a 3.3V bus signal. When using an absorptive clamp device you are doing pretty good if you can realize a clamping at 2x or 3x the actual signalling levels on the interface.

If you have multiple signal lines to protect you can share the energy absorptive device by applying diodes from the signal lines to a single tranzorb thus saving cost. 

If you need much tighter clamping than a tranzorb can provide then you want to consider using clamping diodes on the signal lines that shunt the ESD energy into power rails. Power rails typically have lots of capacitance that can absorb a lot of energy and still clamp a signal line to a safe voltage range. 

You would do well to look at the schematics for some USB type products to see what is typically done to protect such interfaces from ESD. Ferrites, common mode chokes and clamp diodes to supply rails are typical while at the same time not compromising the normal functional bandwidth of the signalling protocol. You could apply these techniques to your SMBus/I2C interface. The USB hub chip manufacturers often have detailed reference schematics that show the types of things I am referring to here.