Another "this is actually the canon ME3 ending thread." You may as well call it what it is.
No, Synthesis is not the "canon" ending. No, there really isn't a "canon" ending. The only ending that's actually consistent and makes sense for Mass Effect, and by consequence the "best" ending, is Destroy. Control is less credible and Refusal is fan service. Synthesis would only make sense based on the premise that the Catalyst, and by extension, the reapers, are "right." If you come to that conclusion and rationalize that "order" is essential, then Synthesis is best for you.
For me, it was rather simple. The Catalyst was a simple depiction of the Hal 9000 situation in 2001: A Space Odyssey. Here, you have an artifical intelligence who becomes too smart for its own good. Thus, the synthetic, such as the Catalyst, turns on the master, organics. So, unless you actually side with that warped logic of maintaining peace through extinction and harvesting, no rational person could conclude Synthesis is a viable alternative (not to mention it failed in the past when the Catalyst tried). Not only that, the entire choice is completely far-fetched and continues to bolster an area of Mass Effect that shouldn't be encouraged, "space magic."
In closing, 100% galactic readiness with Shepard breathing at the end is the perfect ending, from my perspective. Synthesis is by far the worst in its execution and repercussions. One of the themes that made ME3 so compelling was that how various races from all over the galaxy came together to tackle this indestructible threat. Even Javik takes note as this never happened with the Protheans, who enslaved everyone, just like the Leviathans.
Regardless of what choice BioWare thought was the "best," the most consistent was destroy because it promotes diversity, individuality, and a willingness to sacrifice in order to remain "alive" and "organic." Synthesis is really just another form of the reaper solution, thus it contradicts the entire point of the trilogy. Lastly, how can something be the "best ending" when you force unwanted "evolution" on everybody without their consent, when you were fighting for everybody's freedom from the start? That just seems like an indirect slap in the face from my perpective.