If you can afford it's cost, Dragon is a very powerful creature to have, although there are alternative strong, flying creatures such as the lawful griffin. However the griffin has no long range combat attack.

The spell ingredient diamond can produce the spells:

summon dragon,

summon mountain giant and

summon storm giant.

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.
However, 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