Almost any robot can range in difficulty level depending on how simple or complex you wish to make it. If you were to go with the simplest designs possible for each you pretty much have them in order of complexity already.
A photovore can be created using just a comparator chip for control. A line tracer could be built the same as a photovore, except with the sensors facing downward and a couple LEDs to provide light thrown in. Wall avoider on the same level except using touch sensors. Etc, etc. The sumobot would be your most complex base type as it would need to incorporate both line avoidance (same concept as tracing, only reversed), as well as a follower, or rather tracker to find the other sumobot.
You can take the simplest of photovore and making it more complex than a biped, backflipping, wrestling robot. Additional sensors to not only find the light, but look for a specific type of light that is best suited for the solar charging circuit that recharges the battery pack. Memory for the photovore to know which locations held the best light sources at different times of day, month or year. It can get quite complex.
In a nutshell, there are robots which are inherently more simple by convention, but that doesn't mean that a robot of that type is less difficult than any other type. It is all about how much you want to put into it.