ShaderToy shaders work with minor changes. If you have no prior experience with any of these approaches, Processing can probably get you up and running with shaders the fastest.
ShaderToy shaders work with minor changes. Currently shaders don't work in the p5.js Web Editor, and setting up a new JavaScript project can take a bit of practice. But if you've written any JS before, best to start here.
ShaderToy shaders work with minor changes. The process of loading shaders in three.js is unfortunately a little complicated, but it's boilerplate code that doesn't usually change. The shader code itself should work the same as in p5.js—both are WebGL.
ShaderToy shaders require intermediate changes. Shaders in oF work a little differently than in the above approaches, and modifying them may require slightly more understanding of what's going on in the code.
TK
ShaderToy shaders require major changes. Unity is the odd one out among these approaches, because it uses a different shader language--CGFX instead of GLSL. While it's possible to modify ShaderToy's GLSL shaders for direct use in Unity, they'll only work in OpenGL mode; that's impractical for most Windows desktop apps, including for VR. So instead, use the included converter script to convert the shader language from GLSL to CGFX, which Unity can use with both OpenGL and DirectX. (More complex shaders will require additional manual correction.) The good news is, if you can get the hang of this process, Unity offers the friendliest overall approach to shader writing, and also the best documentation.