Which one should I download?

Standalone application: Use this if you use video editing software that doesn't support the plugins offered, you want a drop-in replacement for ntscQT, or you really need fine-grained control over interlacing.

If you're using Kdenlive, Blender, the free version of Hitfilm, Final Cut, or any web-based video editing software, you'll need to use the standalone application.
OpenFX plugin: Use this if your video editing software supports OpenFX plugins.

OpenFX-compatible editing software includes DaVinci Resolve, MAGIX Vegas (nee Sony Vegas), Hitfilm Pro, Nuke, and possibly the paid version of Filmora.
After Effects/Premiere plugin: What it says on the tin. Use this if you edit using the Adobe suite.

System requirements