I, too, have been on both sides of this equation. Here’s my take on things:
Plain text in the body of the email is the safest and easiest method. It won’t offend or annoy anybody. All the content is there, but the presentation is drab and uninspiring. If you’re going to attach your resume to something, put the plain text in the body, also.
Microsoft Word gets mixed reviews. Many managers (myself included) detest Word resumes because they always seem to be for the wrong edition of Word, the files are large, and Word is a security nightmare, prone to viruses. I would never send a resume in Word unless it’s specifically requested. Why take a chance on pissing off someone you need a job from?
PDF also gets mixed reviews. They preserve the format perfectly, but the PDF reader is flaky, and there are so many variables involved that if you don’t know what you’re doing you can screw it up. I’ve gotten PDF files that I can’t cut and paste from, PDF files with printing disabled, and PDF files that crashed the reader. When they work, they’re fine. When they don’t, they’re beyond annoying.
RTF is fancier than plain text, simpler than PDF, more secure than Microsoft Word, and can be opened in the word processing application of your choice. Still, they require opening a program other than the email program, so if you attach one, make sure your resume is still in the email in plain text.
Personally, I email the resume in plain text. In the introduction paragraph, it explains that if you prefer MS Word, PDF, or RTF, all three formats (and plain text) are available on my Web site, and I embed a link. That way, if they pull out my resume a year later (it’s happened!), they know where to instantly fetch an updated copy, too. I’ve never had a complaint about that approach, and I appreciate it when others do it that way.