Quantcast Measure is a audience measurement tool for website traffic. It estimates the traffic to a website and then ranks the websites in order- from most popular to least. That means a website with a #1 rank is the most popular. Quantcast does not measure past the first 100,000 websites.
Majestic Citation Flow is a score which predicts how influential a URL might be based on how many sites link to it. A Citation Flow score is then given to a page, or website, between 0 and 100. A website with a Citation Flow of zero has very few links. On the other hand, a web page with a Citation Flow of 100 has a large quantity of back links.