Dragon, mountain giant and storm giant are supposedly the top creatures in each alignment,
so all are powerful in their own ways.
The mountain giant's indiscriminate punch can cause as much injury to your own local troops
as to the targetted enemy, and without a long range attack, it puts this creature last on the list.
Whilst the storm giant may also be slow, the range of it's long range combat throws,
and the amount of damage the projectile produces makes it formidable.
the dragon comes out on top
because it flies, it travels fast, and has a long range attack,
all three qualities of which easily compensate for it being the most vunerable