I use whatever format the HR department or recruiter or whoever says they want. If that’s Microsoft Word, they’ll get a Microsoft Word version, using default fonts that are guaranteed to be available in their copy of Word and supported on their printer, which also increases the chances that it’ll look ok instead of getting munged into a mess of line-breaks, widows, and orphans.
The original, though? ASCII with line breaks / letting the font default to the reader’s browser’s / email client’s default font. If the HR person or hiring manager uses reading glasses, I don’t want to mess with that, and if (as $DAYJOB’s HR department seems to have done when we last hired somebody) the HR department is feeding the resumes through some ugly standard-format-transmogrifier, any additional formatting I put in at the beginning is likely to mess up the output.