I believe the best order depends on your team's formation.
As you said, each dragon has a special feature:
Maliz: Chain damage
Vengar: Fear
Bruton: High self-healing
Greedok: Intense hex
Vilear: High mana drain
Crultor: Root
Despor: Skill debuff
Since dragons inherit the special feature of those that have already been killed, I recommend that you leave the ones that bother your team the most for last.
For my team, for example, Bruton's self-healing and Vilear's mana drain are the most annoying, so they are always the last ones to be killed.
This is the order that my team kills:
Maliz - Crultor - Vengar - Greedok - Despor - Bruton - Vilear
We tested different orders and this is one that worked very well for us. But as I said, each team will have its own preferences. I recommend that you try a few different ones until you find the best one for your team.