What is the most popular service that was supported by almost every early computer networks? In your opinion, what's the reason for that service to be most popular?

The most popular service supported by every early computer networks:-

Telnet

SMTP (Simple Mail Transfer Protocol)

Reason are as following:-

Telnet:-

When there the number of internet using people was very less They used to get connected with LAN and Telnet.

It was very useful for bidirectional interactive text oriented communication.

SMTP:-

SMTP (Simple Mail Transfer Protocol) introduced in 1982 and was one of the most used protocol for e-mails.

It is still used extensively and It was used for very long time to send and receive email messages for a very long time.

Hence, these are the computer network protocols that are being used as the popular devices.