[DGDobrev]

In my eyes, the Bone Dragons are the best for single-stack army and Red Dragons for 5-troop army, even though I used a single Black Dragon stack throughout most of my Mage games.

Black Dragons are good if you want to stop the enemy hero's spellcasting, do item upgrades and suppressions (since black dragons can't be hit by spells) or deal with enemies who inflict mainly magic or fire damage. The downside is that they can't be ressed and there is no way to boost them with Stone Skin.

Red Dragons have slightly lower resistances and health, but they can hit a number of enemies in an indefinite straight line and can be resurrected by the Inquisitors. They can also act as tanks with Stone Skin and an occasional Heal.

Trolls must not be underestimated as well because of their overpowered regeneration ability, but their usefulness usually ends when the player gets a good army and puts his feet on the ground as he/she should. Another usefulness lies when fighting some underground bosses (Spider, Driller, K'Tahu), because with Stone skin on they can shrug off the damage and regen in the very same turn.

Bone Dragons are quite cool too. They inflict poison damage, which is great - few enemies other than undead have resistance against it, and can also cut through Dragon lines in record time with their poison cloud ability. The other positive is that they can be resurrected with Elvin, while the other dragons (besides black and bone) can only be ressed by Inquisitors.