Have used all of the above, and two axes, which by the way works just fine.
One guy positions the cutting axe, the other drives it home using the second axe as a sledge.
Never liked the chainsaw method, leaves to much bone fragment in the meat.
Makes a nice mess in the saw's clutch housing too.
The reciprocating saw worked well too, but you need the right blade, coarse, and long, or it will plug up.
The hand saw I like, is the newer one that cuts both directions, like this one:
We use it mostly for cutting feet, head, and cross quartering cuts at moose camp, but we use it for spine split at deer camp, works fine.