Hi this is more art than science what I recommend is brainstorming a few and then using a free tool like redacted that scores your subject line (aim for over 90) and gives you recommendations. My suggestion is to not send all emails with the same subject line you will want to test different subject lines so you can identify which is getting the highest open rate...you are aiming for above 20% open rate. The higher the better. Once you have 20% to 40% open rate you will move on to optimizing your opening line and CTA to get more responses.

I do relatively deep research about the founder and company before reaching out so usually have a highly tailored email subject. of course this doesnt work for high volume strategies using AI slop to try and do the job for them.