What do I mean by "best"? Here, I mean highest possible living standard; most freedom of religion/conscience (or approximation thereof); vibrant science and philosophy, including vibrant political debate and high(ish) levels of participation (so "republican" government, etc); most gender equality and sexual freedom (or approximation thereof); and generally things that we tend to value in our modern society... or those that are plausible here, given the parameters.
Those parameters (and "approximation thereof") referring to the other condition -- "medieval". By which I mean, that which was possible, in pretty much any human society or civilization you could point to, provided it was from over six centuries ago. This means no tech that was not commonly available at least somewhere circa 1400 CE or earlier (eg no printing press, caravels, etc); it means any economic or political system must be similarly antiquated (so, for example, you could utilize Song or Yuan level bureaucracy and/or Italian style banknotes as used in the 14th century, but nothing as effective as Fractional Reserve Banking); ditto for concepts of science, gender, or what have you.
Basically, what I'm interested in here is how "utopian" or just "modern" a civilization/power could be without utilizing actual modernity. Any thoughts?