Emails.
07:47pm. During the early days of the internet, computer geeks mostly used just two means of communication: emails and Internet Relay Chat (IRC). Things changed with the launch of private messaging software like Skype, WeChat, Messenger and WhatsApp. However, I still prefer emails for two reasons.
First of all, emails are always at least a paragraph long with a subject line because it is ridiculous to send five different emails when you can send only one. This makes it possible to prioritise a conversation over the other as you know who the author is and what the email is about. This helps maintain a clean inbox -- something a normal user may have a hard time doing on WhatsApp, where messages flood the list of conversations and notifications. Time is the only universal currency and it better be utilised efficiently.
Secondly, emails are more trustworthy. People lie and I firmly believe that it is human nature to lie. Even babies and some other species, close to sapiens, lie. Consider you're talking to a friend and he lies to you. Now, since the conversation hasn't been recorded, your friend might deny details in the statements -- or even all the statements in whole -- he said a few minutes ago. This is not a possibility with emails and IRC. People don't tend to lie or fear saying lies when their statements or activities are being recorded -- probably one of the reasons why corporate offices still use emails. Once you send an email or an message on an IRC server, there is nothing you can do except apologise or better be safe by being true and loyal next time. Even professor Richard Wiseman claimed this. One may argue that this can also be done by messengers. Well, no! Because most messengers have features to delete messages, plus I have not seen any person who regularly maintains screenshots of chats.