Synthesis is not the worst ending. To me, that's the control ending. I'd say none of the endings are particularly paragon/renegade. Well, I guess destroy is a real renegade ending. Accomplishing the mission no matter the costs. Paragon however.. is definitely not control, or synthesis.
If I had to rate from worst to best, the endings would be in the order: Control, Synthesis, Destroy.
Refusal/rejection is actually my favourite ending when selected, demonstrating the same sort of indomitable spirit Shepard had in the conversations with sovereign and harbinger. But the outcome of the refusal ending.. being wiped out.. well, that doesn't sit well. However, that one could quite possibly be the best paragon ending out of all of them. Just doesn't end with a victory for our cycle.
By the way, after talking over the possible solutions with the starchild, you can access the refusal ending by saying none of the 3 options are good enough. You don't *have* to shoot the starchild.