The answer is that there can be no single proper indentation character for every situation. Formatting using characters is inflexible and can cause conflict when different styles are used within a team.
The only method to format code flawlessly and flexibly with different formatting styles is to do it virtually, that is, without any indentation characters. The only code editor I know that supports this though is the one used in the sample below:
To demonstrate virtual formatting, the screenshot below is from an XSLT editor* that uses this indentation method (there's also a short video here). Every character in the XSLT has been highlighted in yellow, for illustrative purposes, to allow the only tab or space characters in the content to be seen clearly. The code indentation is handled by the editor's rendering system adjusting the left margin (which has a white background).
The only leading space characters precede the Books lines, because this is literal text content, not code, these space characters must be preserved.
With virtual formatting you choose the indentation width to suit the environment and indentation style without affecting any characters in the source file. You can even set the indentation width to 0, if you need a flattened view of the code as shown below:
To contrast this with space character formatting, the same XSLT opened in an editor without virtual formatting is transformed by that editor's auto-formatter to this:
The larger blank yellow blocks in the screenshot above clearly show the space characters added by the formatter of the conventional editor. Unfortunately, these now can't be distinguished from real content so the XSLT would have to be modified to correct this issue.
XSLT is possibly an extreme case, but this principle holds true for many programming languages: Characters should be used for content and an alternative method sought when it comes to formatting.
**Disclosure: The XSLT Editor with virtual formatting was developed by my own company*