The best solutions that I've seen utilize a wholistic approach to reforestation. It makes no sense to spend a dollar a tree to plant, and then the very people who cut those trees down in the first place return and chop them all down again. Unless you are addressing the root causes of deforestation (often poverty), temporary solutions do nothing. Organizations that are addressing food security, economic empowerment, and reforestation have a much better track record of long-term success.
Three organizations that I support are Plant With Purpose (out of San Diego) Eden Projects, and Harvest Craft (also based in Southern California).
https://plantwithpurpose.org/
https://edenprojects.org/
https://www.harvestcraft.org/
Harvest Craft is a very young organization but is doing a lot of good stuff, particularly in Haiti (one the most deforested countries in the world). Eden Projects (not to be confused with an organization in the UK with the same name) has planted over 200 million trees all over the world. Plant with Purpose has planted over 30 million, but the significance of what they are doing in communities cannot be counted simply by the number of trees, but by entire watershed restoration and community development.
I used to think in terms of just whomever offers to plant the most trees per dollar. Now I realize how short-sighted that is. If education and food security are not a part of the organization's agenda, they are just going to plant trees and watch them get cut down again. I also see the wisdom in people working one watershed at a time. You've got to plant all the way from the top of the watershed to the bottom, and get buy-in from everyone in the communities that will be effected if you really want to address issues of hydrology and fertility.
If I had ten-million dollars to give, I'd give 90% of it to PWP and the other million to Harvest Craft. And then I'd ask for updates and reports that demonstrate where my dollars went and what communities were helped. In 3 years, I'd go visit these communities and see for myself. In 20 years, I'd go back and see rivers running that once were dry, fertility restored where once soils were depleted, and lives changes where once there was despair.