Im gonna have to vote for Skellies.
There leader is a magician, they have two dragons, a navy, the Crownies would probably be afraid of them and, most importantly, everyone they kill would become a new recruit
The Crownies underestimate their soldiers (an entire catapult against a small barricade, a large ballista against a single horserider) and overestimate their knights (sending one to rescue a soldier from a fully manned prison wagon, sending a single knight to rescue one of the most important people in their nation from a large tower)
The Dwarves are vertically challenged
The Orcs are pretty good but most of their siege equipment would fail (see below) and they seem to be attacking both the Dwarves and the Crownies.
Another matter is who has captured who. The Orcs have captured a Crownie and a Dwarf. The Skellies have captured a Crownie. The Crownies have captured an Orc. The Dwarves are yet to capture anyone. The Skellies are yet to be caught.
Plus, there is the matter of moveable machines: the mine defender could never move fast enough to cut up Orcs with a single Dwarf pushing it with his short legs
The Orcs assault wagon would move fast but as soon as you try to stop you end up with a heavy wagon and an impaled giant troll. Orcs battle wheel would go backwards (if anywhere). Orcs seige tower has nothing to pull it. Neither does the Crownies ballista or the catapult. Skellies have only two land weapons and they are: the prison carriage (which has three horses pulling it and has a support to stop the horses dying on the spikes) and a barricade (which doesn't need to move, all it has to do is stop others moving)
my two cents