Short answer: Ireland.
Becoming a citizen of a new country where you’ve chosen to make a life for yourself is an important moment in many people’s lives. Additionally, how a country treats its immigrant population, eventually recognizing a person as part of the fabric of society and granting them the rights afforded to the rest of the citizenry, is also very important. Some places are notoriously difficult to become citizens of, such as Austria, Switzerland, Germany, Japan, the US and a lot of Gulf countries. But when it comes to the rest of Europe, there are some interesting statistics.
Spain and the UK granted more citizenship than almost all other EU countries combined in 2013, the latest year where data is available. However, in relative terms, Ireland granted the most citizenship of any EU country in 2013, according to Eurostat. Ireland acquired 5.3 citizens per 1,000 residents in 2013 while Sweden came a very close second with 5.2 new citizens for every 1,000 residents.
You will find more statistics at Statista