I'm using this ReCaptcha:
https://vborg.vbsupport.ru/showthread.php?t=315964
But for me, what's really killing the spam bots is this:
https://vborg.vbsupport.ru/showthread.php?t=132482
I replaced all the default images it comes with, with over 200 manually created images, matching my board's niche.
I noticed the registration attempts drastically dropped via the StopForumSpam logs. Because they couldn't defeat that verification page.
Not only did I add over 200 images though, I also made it so there's over 24+ options to choose from.
Quite difficult for someone who doesn't know the niche.