PGP/GPG is the best, in my opinion. With them the sender just makes a document any ordinary way, then uses regular PGP/GPG tools to sign the document with your key, encrypt it with the recipient's key, ascii-armor it, and then just attach it to any ordinary email. No changes whatsoever, in the actual email clients. The recipient detaches the document and decrypts it and verifies the signature with regular PGP/GPG tools.