Is the voltage rating on a light bulb the rms value or peak value?

It's the RMS value. A 120 volt lamp (light bulb) is rated according to its RMS voltage. Just like appliances in the home are rated at 120 volts (like your fridge, microwave and toaster), or 220 volts (like your clothes dryer). Note that these appliances will have to stand up to the peak voltage on the AC line. Naturally. And the peak voltage on an AC line is 1.414 times the RMS value of voltage. That means in a 120 volt AC line (120 volts RMS), the peak value of the voltage will be 1.414 times the 120 volts, or right at about 170 voltspeak for each cycle.