Ukraine, the biggest challenge for the European Union. The European Union has experience in integrating countries from the socialist orbit and even former Soviet countries.
But there is no point in doing it with... a country that is suffering the worst armed invasion since the Second World War. Ukraine applied for EU membership in February, and was granted EU status in June 2022. In December, EU leaders decided to start negotiations to start accession negotiations in June 2023, with a view to joining the EU in 2024.