Never heard of AROS...
ReactOS is bloated by definition of replicating windows, isn't it? It probably uses less than main windows, but it is just my opinion that the 1990-1997 period preceded some of the real bloat added due to the massive Moore's law gains after that with the GHz races.
OS/2 ... seems obscure to me but I never used it. But NeXT and Beos had revolutionary capabilities, I think OS/2 was basically just preemptive multitasking for Windows. Is that right?
BeOS, which I've never used, is probably also a good starting spot. HaikuOS probably has more oomph behind it community wise.
It is very wrong assumption!