I believe if we are to look from a very pro-Ukrainian but still somehow objective perspective the proto-Ukraine would be Zaporozhskaya Secha - a semi-authonomous Cossak statelet existing on a very small territory around lower Dniepr River. It did exist in some form between late 15th and late 17th century as a buffer zone between Poland, Ottomans and Russia but mostly Russian-controlled. It disappeared around 1770s.
Between 15th and 18th centuries, sorry.