You're selecting an OS for all the wrong reasons, to be brutally honest. First select the applications you want to run, then choose hardware, OS, and coolness factor. Chances are quite big that you will not find Linux to your liking as well. You can tinker with it to your heart's content and some internals really have an Amiga-like feel to them (even if they're much more elaborate) but unless you like to do that all the time, it is not really that different from the rest. It just crashes less often---although I have to admit that Windows 2000 is already quite stable.
The only idea I can think of besides the OSes you already mentioned is SkyOS, but there you have to work with an appbase which might (or might not) be big. I am not that interested in these things anymore.
I think it is time for you to come to terms with the fact that something like the Amiga is no longer available nor feasible. Well, if you insist, you can use AmigaOS4, or MorphOS, or perhaps even AROS, but you already indicated you're not that keen on those. It took me a few years, but when I tried AROS a year ago, it felt almost like sacrilege to have something that (relatively) simple run on hardware nearly two orders of magnitude faster. My entire usage pattern has changed too: I cannot do on an Amiga what I now do on the PC anymore.
Of course the above is invalid if you don't care about apps; then SkyOS would be my first non-Windows, non-Linux, i86-based OS of choice.
