In the AI governance space (specifically research; I'm not familiar with advocacy), I think GovAI and RP's AI Governance & Strategy [edit: now IAPS] team convert marginal money into research very well. I think this mostly because I like their teams and track records and (last I checked) they say they want to hire more but are funding-constrained.

But I wouldn't be surprised if some alignment or advocacy thing is predictably better.

I wouldn't be shocked to find it's a scrappy piece of anti AI advocacy like the AI pause work. Like maybe they get a big win and then it's been a few $k and very large impact. 

https://manifund.org/projects/holly-elmore-organizing-people-for-a-frontier-ai-moratorium 

Family Empowerment Media seems more cost-effective than GiveWell. They look especially promising from a cluster thinking approach.

• Having control of their pregnancies also helps women according to a capabilities approach to welfare
• Preventing unwanted pregnancies is a great opportunity in terms of animal welfare, the opposite of the meat eater problem
• Is ~2x more cost effective as GiveWell's top charities according to Founders Pledge, possibly even higher according to Rethink Priorities
• (speculative) There's a small possibility GiveWell might not want to fund them no matter their cost-effectiveness. GiveWell's mission seems to be to move as much money as possible to effective charities, not to donate the marginal dollar to the most cost-effective opportunity.
  GiveWell might worry that funding family planning would make it harder for them to get contributions from some very large donors, and hence hurt their mission. (This is very speculative and unlikely, but GiveWell has been in talks with FEM for more than a year, and hasn't published anything more than this, despite FEM having a clear funding gap. And I get the impression that many UHNWI are extremely pro-life and worried about declining fertility rates)

If you agree you can donate.