Dragons with class levels can be quite frightening if done well, even just the Dragon classes that the Draconomicon has. The Unholy Ravager of Tiamat, for example, with its cleric spellcasting and semi-Divine Blast salient ability. Even just a straight Divine Rank 0 (which can also be gained through the Dragon Ascendant class) provides a number of rather hefty boosts to a dragon, what with the immunities, hit points, speed, AC bonus, DR and so on.