Which desktop environment is the most stable in ubuntu?

The most stable is the official unity, I have used several desktop environments before and after, if you ask the stability of the words in order of judgment should be unity -->gnome3 -->kde5, I am now using the kde, gnome tends to be business themed style, kde is more energetic, customization is easier. I hope this helps. Each of these has its own strengths. I've never used any of the others and I can't comment on them.