Preheaders and subject lines are plain text, so unicode is the only styling that survives every client. A single styled word can lift a subject without tripping spam filters.
Generate the word with Font Style Generator, paste it into the subject, and keep the rest plain. Test in Gmail, Outlook and Apple Mail before sending to a list.
Do not style the whole subject. Deliverability tools treat heavy stylisation as a risk signal.
Written by FontStyle - more at fontstyle.io.