Under defense, mind blank is a premier spell. It also serves double duty under obfuscation when coupled with some form of invisibility or shapechanging, because it stops true seeing.
For counter-intelligence, I'm a huge fan of detect scrying. Coupled with nondetection or mind blank, it makes for a nice "you can't see me but I can see you" trick and gives you enough info to launch a counter-attack via teleport.
For action enhancement, arcane spellsurge effectively quickens every spell you cast, and drops summons down to a standard action. However, it is more abusable as a Sorcerer. If you're including spells you can add to your list, arcane fusion and it's greater cousin are extremely powerful as well.
I have to second the shapechange -> elemental weird idea for intelligence gathering. Free action contact other plane is powerful (the weird abilities were reprinted in Frostburn and remained free actions).