This question is really tough. I do not think any colonial empires actually treated their colonies well. There is no evidence or historical account take any colony was treated well by the empire that subdued it.
Most of these colonial empires treated their colonies as slaves.
They never treated their colonies well, so many lives were lost. I do not see how possible it is to treat a country that is under another country well. Most of the worst colonial empires were European power, especially Belgium, British Empire, Germany and France.
This is a difficult question to answer because I do not think any colonial empires actually treated any of their colonies very well, especially when you consider some of the people in the colonies were often treated as slaves. The European colonial era spanned from about the 1500’s to the 1960’s.
All but 5 or 6 countries in the entire world were at some point in history, under the control of another power, mostly a European power. Of course there are a few exceptions to this. For example, Portugal fought to maintain control of its African colonies until 1974. I don’t know how anyone can think any colony was well treated when they were under the control of another country.