What is the most formal method of Business communication?

Through E-mail or official letter on the company letter head. 

E-mails is the best, fastest and trusted means of communications. A personal meeting with the customer is also the best way to commute.

emails, reports, letters, minutes of meetings, telephone calls, faxes, ... sometimes IM messages and text message through mobiles

through email or telephoe.

Business letters are the most formal method of communication. It follows specific formats. and is addressed to a particular person or organization. There different types of business letters :

1. Letters of inquiry
2. Letters of claim/complaints
3. Letters of application
4. Letters of approval/dismissal
5. Letters of recommendations
6. Letters of promise.

The most formal type is the letter one.