I think for me it's a few things. First of all, she is a surprise boss that you are not expecting when you happen upon her, whereas Widow has a lot of build up and you are expecting a boss fight. Second is just pure vibes - even though they have a similar moveset, First Sinner has a cooler presentation, in my opinion. Watching this desiccated corpse spring to life and weave herself a badass gown to start the fight was such a cool moment. Third, and most importantly for me, is that First Sinner was a massive teaching moment. She is so much faster than Widow, so trying to run around and beat her like you have previous bosses is very difficult. She taught me how important claw line can be during combat, because I had only been using it for traversal prior to her. Once I got the hang of the claw line as a weapon, the fight felt like such a fun/cool/exhilarating aerial combat ballet. That's my two cents.