What I like SkyScanner.ru for is an ability to select a whole month as departure and return date. I was unable to find the same on other sites and the feature is not very visible at first. But this is a major feature. Very handy when you are trying to decide when to go on vacations based on cost of tickets 🙂
This might be not the best price , but then you can check directly airline company and other sites with this specific date.