What is the most and least secure way of going about encrypting emails?
There is no difference in security: both use the same cryptographic principles, they just use another method of embedding OpenPGP into e-mails.
I am looking for security and have no idea which to use, though I do know HTML will not work with PGP Inline but will with PGP/MIME.
PGP/MIME has the advantage of properly hiding signatures in metadata when the recipient does not have support for OpenPGP (they might see a "weird attachment", though). PGP/MIME is sometimes not properly handled by mailing lists. PGP/MIME lets you also encrypt and sign attachments.
Inline PGP on the other hand is more visible, and more robust in case of bad MIME implementations. Inline PGP cannot encrypt/sign attachments.
Both methods only support the content of the mail, and do not touch the mail headers (subject, sender, recipient, date, ...).