Adobe PDF file - best I know of, no reported virus attachments or embeds.
RTF - rich text file (word can save your documents in RTF) no reported macro virus attachments or embeds.
DOC - Word Document file - well known for transmitting macro virus attachments or embeds. Lease attractive for your recipient to open, filtered out by some ISP(s)
Here is a list of the warnings my computer give when I atempt to open a .DOC file recieved by email.
(in word)