you want it engine side of the thermostat ideally chris, otherwise it won't read much until the thermostat opens.

if you're only interested in the hot temperature, then after the thermostat would be ok. in the rad would be fine if you put it right near the inlet hose which will be the same temperature as the outlet of the thermostat once it's opened fully. the further away from the inlet hose that you go either down or across will see the temperature dropping quickly