Unless that's an ancient dragon (making it eligible for the Ravener template) your options with the dragon are limited. The best undead templates are either humanoid-exclusive or will remove its ability to fly. If your GM allows the Graveknight template on the dragon then you're in business - it's virtually impossible to permanently kill (similar to a lich), retains its ability to fly, and gains some nice stat bonuses. Even if you can't put it on the dragon, the Graveknight template is probably your best bet just because it's so hard to permanently kill.
which humenoid templates would you suggest?
Graveknight is probably the best due to how hard it is to kill permanently. The only real downside is that it needs bonded armor, largely precluding arcane spellcasters and unconventional creatures. While you might be able to convince your GM into allowing custom barding for a non-humanoid creature, it's a bit of a stretch and he might not allow it. This template noticably more powerful than other +2 CR templates and has no real weaknesses.
If you're okay with huge glaring weaknesses, Vampires are another great choice. The downside is that they're always one round away from being reduced to a smoking pile of ash. There are protective spells, sure, but you're still one caster level check away from death. If you're okay with the risks, there's a big power payoff in this template.
The Oracle is probably the best class selection for any undead minion. It allows you to build with charisma as your highest stat, which for undead creatures results in a big hit point boost. Oracle synergizes well with the physical and charisma attribute boosts that most undead templates have, and is a potent class to begin with.