Actually, there are several Dragons that don't learn Flamethrower:Grass-Dragon has potential: Flamethrower (haven't seen a dragon without it)
Palkia learns flamethrowerActually, there are several Dragons that don't learn Flamethrower:
Latias
Latios
Kingdra
Palkia
Giratina
Sableye yes, Spiritomb though has good stats even if they're not "too good".Palkia learns flamethrower
I voted for Ghost/Dark because 3 immunities and no weaknesses. The only problem is that the BST of the 2 ghost/darks we have are fairly low, limiting their potential.