That said, I don’t think either the link or TI’s index support the claim that Ukraine was “the most corrupt country in Europe before the current situation”.
First, regarding the corruption index — “each country’s score is a combination of at least 3 data sources data sources drawn from 13 different corruption surveys and assessments”. And “the sources and surveys which make up the CPI are based on carefully designed and calibrated questionnaires, answered by experts and businesspeople”. So fundamentally perceptions (as per the title) instead of hard data — and probably different sources across different countries calibrated in some way (it might be sourced somewhere but one only has so much time).
Second, and I think more damning to the statement about Ukraine being the most corrupt in Europe prior to the current events by the referenced corruption index itself. Russia and Ukraine trade places on which is perceived to be more corrupt up until 2017 when Ukraine is clearly less corrupt than Russia. I spent my lunch graphing the data and only feel like it was mostly a waste of my time.
My bottom line based on the data: Russia and Ukraine were roughly the same amount of corrupt until 2017 when Russia distances itself as clearly more corrupt. The rankings themselves are based on expert perceptions rather than hard data. The statement about Ukraine being the most corrupt state in Europe before the current events is only true if you consider Ukraine in Europe and Russia not. Which one could certainly do but considered their shared past during the USSR seems a fundamentally flawed way of treating them.