Having used all of them at one point or another, I should be able to answer this question well.

Windows

Pros:
Loads of first-party and third-party support and applications.

Cons:
Contains Microsoft spyware in Windows 10 that can't be removed. The company acts like you are only renting your computer.

Macos X

Pros:
Closed hardware means heavily tested configurations are available to those who color inside the lines. Well supported OS for said closed platform.

Cons:
For those who color outside the lines and get an after-market graphics card from nVidia, Apple fights you every step of the way. Company makes you WISH you had rented the machine because it will be obsolete within 5 years.

Linux

Pros:
Gives new life to obsolete hardware like those cast-off from Windows and Macos users. Is basically many operating systems in one depending on the GUI library you choose. Third-party drivers for many peripherals.

Cons:
If you don't cherry-pick your GUI library apps you end up with multiple GUI libraries installed, making you need modern amounts of memory, you may not be able to run it on obsolete hardware from Apple or an old PC after all. All drivers are essentially minimally tested third-party drivers or may be closed source if you get first-party ones.

AmigaOS 4

Pros:
If you are lucky enough to have a supported Radeon HD graphics card, you can get an add-on set of Warp3D Nova drivers that support OpenGL ES 2 and GLSL shader support with it.

Cons:
Hasn't been developed by first-party developers for years. Less software than AmigaOS 3 has. Is slower than MorphOS and boots as slowly as a cut-down Linux disrto because it uses React GUI toolkit but most applications use MUI.

MorphOS

Pros:
Better development library stack. Faster, more streamlined design. As a result of the powerful development tools, has first-party GUI apps like Flow Studio, Git and is getting an up-to-date browser and emailer.

Cons:
No shader support yet in its graphics drivers. WebGL will require OpenGL ES 2 support so that can't be supported yet.

AmigaOS 3

Pros:
Supports many old hardware-banging titles that milked the 68k Amiga models for all they were worth that require emulation on all the others. Can impress programmers with ability to write code that takes 1-2% of a GiB of RAM.

Cons:
Graphics card support is a third-party add-on. USB is a thrd-party add-on. TCP/IP stack... well, you get the idea. WHDLoad is required to get most older software to run because hardware-banging code is not portable. Lacks decent support for its own chipset, making hardware-banging a necessity.

I could go on but this is a general gist of things. I look forward to seeing your comments.