Shades, Greater Shadow Evocation, Greater Shadow Conjouration come to mind; just because they are really flexible (although occasionally some of the things you can do with them make your head hurt if you think about it too much - Shades can conjour a Magnificent Mansion .... So what happens if someone with Spell Resistance goes inside (and beats your caster level check) while people are in there? Is the food serverd healthy to eat?) and one thing a Sorceror lacks is flexibility on the spell list.
Greater Planar Binding can be good ... if you have Magic Circle Against Good, Magic Circle Against Evil, Dimensional Anchor, and some Spellcraft. Draw a diagram (take 20) in a secure location, anchor it, and summon up some help - how useful it is depends, a lot, on your charisma score and your DM, but a 16th level Sorceror (who took Greater Planar Binding for the first level 8 spell) can summon up a Planeatar, which is a 17th level Cleric..... and thus has 9th level Cleric spells available (True Ressurection for the material components - at 25,000 gp.... after a day for it to prepare the spell for you.... and an opposed charisma check against a 22 cha critter). Just make sure not to anger it too much, as it might come hunting you up....
Gate and Wish have been mentioned.
Disjunction is handy against spellcasters.
Shapechange can be nice, as it gives you access to a boatload of abilities, depending on the form you take on.
Time Stop is always good for when you just need an extra moment.
Dominate Monster can really ruin your DM's day, if he isn't prepared for it.
Most the higher-level spells are good, actually. What's the best is a different question....