It certainly isn't the worker, whose labour becomes cheaper every year (there hasn't been an increase in real wages for over a decade now in the US). The consumer certainly benefits from lower prices. However who arguably benefits the most is - as always - the proprietor of the production factors: ie. Amazon, which supplies the jobs, the technology, owns the storage facilities, etc.

Now: As a net result over the whole economy, the efficiency gains achieved with these new work contracts are beneficial. However these gains are unequally distributed between the proprietors (ie. Amazon and it's shareholders) and the employees. While incomes for many employees have stagnated for over a decade, the shareholders of Amazon keep getting richer (https://www.google.com/finance?chdnp=1&chdd=1&chds=1&chdv=1&...) creating more inequality. This is why we need a universal basic income policy. To allow everyone at the very least dignified living conditions.