Swamps or bogs may be more efficient on a net fixation basis. Anaerobic deposition may better preserve the carbon as well.

I'm not immediately finding a comparison of various ecosystems, though there's this on Indonesian swamps:

https://www.sciencedirect.com/science/article/pii/0045653593...

And:

"Peatlands only cover about 3 percent of the Earth but they accumulate more carbon than tropical rainforests," says biogeochemist Nancy Dise of Manchester Metropolitan University in England. "In terms of sitting there kind of quietly year after year packing away massive amounts of carbon, nothing tops these peatlands."

https://www.scientificamerican.com/article/peat-and-repeat-r...