A very good question - in my experience the key to good control is placement and mass of the temperature sensor.
You can use a thermocouple or an RTD (resistance temperature device) depending on your temperature range.
But in most factory process controls the biggest problem is how to make sure that the sensor is measuring the ambient temperature, or the temperature of the object being heated. If any radiant heat energy falls on the sensor, you will get meaningless temperature readings.
In some cases a simple reflective shield will allow the sensor to measure the ambient temperature.
Sometimes the sensor is embedded in either the object or in a dummy object with similar characteristics. After all your goal is to control the RIGHT temperature, not just any temperature.
The sources below give a typical easy to use PID controller and a high power SSR.
The controller has an autotuned function - this is very key because tuning a PID controller can be very frustrating. You can end up with wildly cycling kooky results unless you are experienced.
Remember that a Proportional Integrating Controller is a big step up from a bang-bang on off relay type controller.
Proportional just means that it can send a small milliamp signal to the Solid State Relay allowing it to proportionally adjust the heat output of your resistance heaters.
Integrating can be interpreted to mean that the PID takes into account the amount of heat you have already pumped into the system. So it can avoid shooting a big pulse of heat into an oven which it already knows is pretty fired up already.
And Derivative means it watches the rise or fall of the temperature curve to prevent overshooting and undershooting. This keeps the temperature from varying wildly. Maybe! Depends on if you know how to tune it.
Give these folks a phone call, they offer free application consulting, even for small projects.