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There are a few available, which one is the best?
Visual Composer has been around for a long time now, but it's well supported. It's the go-to page builder for a lot of people, and is included in a great many pro themes (especially on Theme Forest).
Cornerstone, built by Themeco (the development shop responsible for the X theme) is a fast, and 100% inline page builder
Beaver Builder offers a theme with the plugin baked in, or the standalone plugin (well, you get both when you purchase a license). They also offer a lite version in the official WordPress plugin repository, but it's a pretty cut down version, and doesn't give you enough functionality really.
It's a nice page builder, and doesn't litter your code with shortcodes (which would make it difficult to remove) like some other page builders.
The Divi Builder written by Elegant Themes was originally just a part of the Divi theme, but has since been separated out into a standalone plugin (thankfully, because cleaning up all the shortcodes if you wanted to switch themes was a pain!).
There's quite a bit of flexibility for building layouts and pages, including (currently) 46 content elements.
Being Elegant Themes, you don't just buy the plugin, you buy access to all of their themes / plugins on a yearly basis. As we're talking about the plugin here, you'd need the developers licence, which currently runs at $89/yr.
Parallax Gravity offers a free version via the official plugin repository, or a pro version at CodeCanyon.