Google, Facebook, or unicorn has the best engineers?
Don’t you know by now? Any poll on blind seeking best/most prestigious/intelligent Google will win. Any seeking worst/least prestigious/dumb Amazon will win. That’s all you need to know
The best engineers are where the best products are. Count how many products & features do you use from each company and you'll find your answer
Depends a lot more what team you join within each than which of the two you join.
To be honest both companies have their share of smart (and dumb) engineers. Google probably has an edge.. for every 2 celebrity engineers at Google (i.e. Jeff Dean, Ken Thompson) Facebook probably has 1 (i.e. John carmack)
I wouldn’t say Unicorns have any visible upper hand here. Really location and group dependent. Generally FB, Google will have better concentration of top talent due to better ability to support and retain them.