If you want to run a "Classic" Amiga, then the latest AmigaOS you can get is 3.9 (which is really 3.1 with bells and whistles, but they can be quite useful!). For the "next generation" Amigas you have a choice of AmigaOS 4.1, MorphOS or AROS - each of which has good sides and bad sides - but for Classic it has to be OS 3.9 (or 3.1 for an unexpanded A1200).
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