hmm.. the greatest european nation it's hard to tell, but i could tell you what nation provided the most important (and probably the best) changes, which is roman empire (laws, latin, culture, and ofc the empire..). the strongest european; i'm not sure, undecided among england, roman empire, maybe even chocolate chip cookie german (probably not) or spanish/german empire (holy roman shit, though not really strong); these are the strongest imo.
edit: lol almost forgot, USSR also