Trees would be great. Right now we're deforesting at massive scale. So first we have to stop doing that, then maybe we can reforest and pull down some carbon, and preserve some biodiversity while we're at it. We probably have to get this done soon, because drought, heat stress, disease, and forest fires are starting to turn some forests into net carbon emitters.

But there are also technologies in development that can remove about a thousand times as much CO2 from the air as trees, for a given land area.