Which species of great ape is the Earth's largest arboreal mammal?

What is the Orangutan, Alex? I'll take "Strange Things You Find in Your Teen's Room" for $400!

The name "orangutan" (also written orang-utan, orang utan, orangutang, and ourang-outang[1]) is derived from the Malay words orang, meaning "person", and hutan, meaning "forest".[2][3] The locals originally used the name to refer to actual forest-dwelling human beings, but the word underwent a semantic extension to include apes of the Pongo genus at an early stage in the history of Malay.

Sadly, the orangutans in the wild are critically endangered with the greatest threat being habitat loss.

https://en.wikipedia.org/wiki/Orangutan

And to define 'arboreal mammal' for those of us who had to Google it, these are mammals that spend a significant portion of their lives in trees. They are adapted in one way or another to safely navigate the perilous environment. There is a very interesting Wikipedia article on 'arboreal locomotion' (movement of animals in trees) which details many of the challenges and adaptations animals make.

https://en.wikipedia.org/wiki/Arboreal_locomotion