[amiga386]

Having used AmigaOS, Mac OS, RISC OS, MS-DOS and Windows in the 1980s, I can say that Amiga definitely had the smoothest experience. Even the most basic things like reading/writing a floppy disk could make all but AmigaOS unusable. I remember thinking about Windows 95 "why is the mouse pointer juddery? It should never be juddery!" when trying to run several things at once.

The reason you don't see lots of "big" apps at once is because memory! RAM was expensive.

If you run a CPU-bound task on AmigaOS, you can use ChangeTaskPri on it, and you can easily see the difference. At its default priority of 0, the mouse pointer is still smooth but other applications start to clam up and starting new things is slow. Reduce the priority to -1 and everything's full speed again. Raise the priority to 5 and you'll have trouble moving the mouse pointer. Raise the priority to 20 and the system hangs...

I don't think you've truly experienced "cooperative" multitasking. Try RISC OS, which literally passes control around between applications and has to wait for them to give it back, unlike AmigaOS which will take it back from them. Any kind of error is an immediate modal window that stops the world. Accessing a disk stops the world. Pressing F12 on the desktop drops to a command prompt and stops the world. You can open a command prompt in a window but it runs at a fraction of the speed of the global command prompt.