Could you support that claim with evidence? Not just propaganda claims from anti-communist agencies.
Not sure how many countries are communist, but Cuba doesn't appear the most corrupt, and North Korea does even appear.
The Corruption Perceptions Index Rank (CPI Ranking) is published annually by Transparency International, a non-governmental organisation. A country or territory's rank indicates its position relative to the other countries and territories in the index.
tradingeconomics.com
ETA I was surprised that denmark was top the corruption list.