By your metric, the most "socially backward" country that joined the EU was the United Kingdom, as its the only one that actually exited so far.
EDIT: regarding your edit - yep, Ukraine is currently a very backwards and crappy country, basically no better than Russia. While Russia is propped up by infinite monies flowing from the ground, Ukraine doesn't have it, so it's much more dilapidated. It's a small miracle that it didn't collapse economically and became Putin's client, just as Belarus had before. However, the one huge difference is that Ukraine doesn't have centuries of imperialistic and autocractic traditions that define Russia - so, it's population is not excited about subjugating other nations, and is ok with democracy as a form of government.
EDIT: regarding your edit - yep, Ukraine is currently a very backwards and crappy country, basically no better than Russia. While Russia is propped up by infinite monies flowing from the ground, Ukraine doesn't have it, so it's much more dilapidated. It's a small miracle that it didn't collapse economically and became Putin's client, just as Belarus had before. However, the one huge difference is that Ukraine doesn't have centuries of imperialistic and autocractic traditions that define Russia - so, it's population is not excited about subjugating other nations, and is ok with democracy as a form of government.