Synthesis was always the 'best' ending in the context of BioWare's shitty writing. It's only a bad ending when you start applying reason and logic, the same reason and logic that refutes most of Walters' dreadful climax. The same reason and logic that identifies why the Catalyst's reasoning is a logical fallacy.
But in the context of BioWare's shitty ending, it is the 'best' one. Because BioWare wanted the entire series to be suddenly about an inevitable war with synthetics. One that cannot be stopped. It will happen forever and ever and ever for dumb reasons that don't make sense. Synthesis is the peace and harmony to oppose this ideology: robots and humans finally together as one.
As said, and like all the other endings, it is stupid. But only because you apply the same reasoning you use to dismantle the awful, thematically inconsistent and unbearably stupid climax. Because BioWare obviously doesn't think their ending is stupid, Synthesis is their best case scenario. Nobody dies. Everybody lives, and are better off for it.