What is the most recent well-attested common ancestor of chimpanzees and humans?
Two reasonably close relatives of the most recent chimpanzee-human ancestor (CHA) are Sahelanthropus tchadensis and Orrorin tugenensis, but both are poorly attested.

One better candidate is Ardipithecus ramidus, which has a fairly-complete skeleton ("Ardi") among other fossils. It's a bit young (4.4 Ma), perhaps, to be a CHA but should be a close relative.

Another reasonable candidate is Oreopithecus bambolii. It is very well-attested. The trouble is that no one seems to know how it's related to humans; 'best guess' is that it's not actually an ancestor but a sister taxon.

(I'm not an evolutionary biologist, nor even a scientist, but I thought I'd leave what information I collected here as there is no better answer at this time.)