Which temperature sensor represents the CPU on my iMac?
They're all valid temperature. It's kind of like asking "what temperature is my car's engine?" There's the oil, water, pistons, camshaft, etc. that all have different temperates. Just monitoring the oil temp won't tell you that one of your pistons has overheated.

But here's the thing, your computer won't overheat. Most (if not all) modern CPUs "thermal throttle" when they reach a limit placed by the manufacturer or motherboard. In this case, that'd be around 100°C for most modern Intel CPUs. Thermal throttling means the CPU will run slower, therefore using less power, and thus not heating up as much. It's perfectly fine to run a CPU at it's max temp for years, it's designed to be used and won't "overheat". If you prefer, of course, you could turn the fan up higher (or reduce the workload), but that won't really have any effect on your CPU's lifespan. 

To directly answer your question, I have Macs fan control, I usually just set it as the first core, but I've figured out that one of my cores is hotter than the rest, and I just use that. CPU PECI seems to be some sort of average or entire die temp, which should be fine in most cases.