A point on "best."
There seems to be a lot of interest, some coming from the tech culture of evaluating charity and its efficiency. This is a good thing and could make a lot of difference. But for an individual that wants to give, you don't necessarily need it. You just need to find a destination for your donation. Try to understand that single organisation.
I like the idea of simple things where the money = thing equation is understandable. For example, I have a distant cousin in the Philippines who builds "houses" for squatters with arrangements with land owners. It started after the storm. He lives locally and understands building. It's about £400 per hut. Most of the cost is labour. Materials are largely donated. The crew are locals that he knows. The recipients are living rough otherwise.
This may not scale to millions, but it works for individual donations. IE, you can find a charity that has the capacity to take your donation. Understand what they do and make an effective choice. It's a much easier problem to solve than the "how to make global donations more effective" problem and is really the problem at hand when you make a choice.
Also..
Why was I doing it? Maybe the donation was the equivalent of an agnostic’s prayer, on the off-chance the supernatural listens to altruism.
I like that. I'd apply an atheist version for myself. A prayer to human solidarity and fraternity, the things I hope for. I don't expect to make these things happen personally, but I can offer my prayer.