Probably the most popular of all the sites, as it is the most widely known. They have a fairly standard PPR scoring format. They also allow late edits. Which means you can change a player/edit your lineup up until that player's game starts. Whereas other sites lock their lineups 5 minutes prior to the first game going off on that slate. The late edit option is what draws me to Draft Kings, that and there are just more contests because there are so many more people that use it.
Probably the second most popular site because it has been around the longest. They offer a .5 PPR scoring format. Like Draft Kings they offer the late swap/player edit. However, all of their contests do not have that, so you have to make sure you read the rules for that contest prior to entering.
This site has grown widely in popularity over the past two seasons. They have the same PPR scoring format as Draft Kings, and they have the lineup lock which allows no roster edits one minute prior to the first game starting on the slate. One big draw for them is they offer a subscription package that allows you to enter contests that do not have a rake, and they are the only site that offers this option.
Yahoo DFS is obviously popular as they already have their own little "cult following" with Fantasy sports. So naturally most of the people that play season long on Yahoo, also use it for DFS. They offer standard .5 PPR scoring like Fanduel.