Plain text (ASCII)

Whenever possible, just avoid using formatted text: using plain text (either ascii or .txt format) guarantees complete access for everyone, regardless of their software, their operating system or the computer they are using. In your emails, if what is important to you is the content and not the formatting, send the text directly in the body of your message instead of sending it as an attachment.
Plain text can carry no virus, it is extremely light and can be easily used to create tables (with tabs or commas) which any software is able to read.