What do you think offers the most Amiga like experience?
Perhaps AmigaForever gives the shortest path to get Amiga experience. (you can easily enchance it via AmigaSYS or Amikit package)
Then upgraded real A1200 might be nice to tinker with real state of the art Classic Amiga.
MOS and AROS will enable to test modern stuff easily.
For AOS4 you need extra HW, but perhaps it most resembles AOS3.x.
I have some experience of AOS68k,AROS,MOS and AOS4.
I still need real 68k Amiga for things like fully working Deluxe paint, genlockable output etc. (I have A4000D/T, A600HD etc...)
About next gen:
I've tried to use AOS4 as my main system, but so far it has required too much work to make it work perfectly (ok, if you have the time to just to tinker with it, but if you need to produce something...) AOS4 ships also with real 68k kickstart files and it's the first varian to head to multicore support. AOS SW development community seems very active, but perhaps it's just more visible. I have SAM440ep-mini and it's a little feels a bit too slow for serious use, but I think it's most mature platform for AOS4.
MOS seems the most mature NG variant. I believe that with it you end up faster in doing things rather than just trying to set it work perfectly. I definitely should use it more (I have MacMini HW).
AROS, when I last tried it, felt raw. I think with AROS you need most work to make it nicely running. But also for AROS, I should install latest version to try it more. For AROS I have 1.7Ghz sempron setups (desktop and laptop, but for laptop I need to figure out how to get AROS back on it, after DVD drive is broken and it does not boot from USB). AROS seems to need most HW performance to run OK. AEROS (broadway X ?) might one day prove to be very interesting AROS distribution, it should enable running lightningly fast AROS+AROS apps+linux apps+68k apps, just via clicking on any of those application icons (it runs hosted on linux kernel, other NG variants are fully native).
For any NG OS you should get above 1Ghz CPU.