If the dragon is powerful enough to get these, these are great ones: greater invisibility and haste. I've always found that protection from energy is a no-brainer, because the dragon in question is almost certainly going to get attacked with spells of some energy type other than his own. If you've got a very powerful dragon, greater dispel magic wreaks havoc with a party that just finished buffing up. Given the range of a fireball (or similar spell), if you've already got an invisible flying dragon, then spells of this sort make for a wonderful way to soften up a party from a distance, and it also gives the dragon clues about what sorts of protections the party has. Finally, if you're talking about a dragon that frequently gets into melee, stoneskin is hard to beat (unless you're in a very high magic world).
Dave
Dave