This is an interesting article recently published in Forbes. The author gathered data from Glassdoor.com, to rank companies. Glassdoor.com is a website where employees make comments about, and rate their company, and can even post their job title and salary range. Keep in mind that the author is not a statistician, and his analysis is based on user-generated content, which, like all reviews, can be fake. Yet the author removed companies with less than 10 reviews, to make this table more robust. So if your start-up is not listed, it's not because it's not in the top, but because there are too few ratings to make a sound conclusion about it.
The picture below show companies at the very top. Read the original article for full list and comments by the author, and to check out if some important companies are missing. Or you may create your own table, scraping Glassdoor's reviews (or hiring an intern) to produce your analysis.
Below is an extract from Glassdoor (screenshot), showing summary stats for two companies. Why not create your own API for company rankings, and even sell your collected data in bulk, to interested organizations? That would be a nice job for a data scientist, with potential revenue stream!