Tabs, and on document format in Visual Studio I have my settings such that spaces are converted to tabs. I even often enable viewing whitespace markers while editing. The times when it does happen that you need to correct indentation yourself, deleting individual spaces is too annoying.
Not sure this holds much relevance anymore, as I have all IDEs and Editors setup to convert from one to the other, plus most of these offer the ability to copy either as spaces or tabs.