I'm not sure if you're asking the order to daisy-chain your reactors, or which one is the most important. If it's the latter, then the carbon reactor is the most important/useful. Assuming you have a phosphate issue, then the GFO would be the next most important/useful, and the bio-pellets would be last.
If you're daisy-chaining I don't think it matters much. Some would say put the carbon after the GFO to catch any GFO particles that leave the GFO reactor.