My sorcerer PC took Chain Lightning as his first 6th-level spell. It's been tremendously useful in our campaign, mainly because we have been in a lot of close-quarters battles. Unlike area spells, CL lets you pick and choose which targets are hit, which is great when baddies are swarming around and between your allies.
After that, I took Disintegrate. It's one of the few save-or-die spells that works on constructs and undead; besides that, it has great utility use, as it can create a shortcut through almost any wall, floor, or other obstruction.
Next will probably be Greater Dispelling, since I didn't take ordinary Dispel. If not that, then perhaps Prismatic Eye from MoF. (It acts like Prismatic Spray, but projects a ray of a single random color once per round, and each color is only fired once.)
I might have taken Mass Haste, but that seemed redundant since I have Haste as a 3rd-level spell. Instead, I took the Chain Spell metamagic feat, from T&B. So I can cast Chained Haste with a 6th-level slot, without needing to waste another known spell on it. The additional benefit is that the feat is useful for lots of other spells too.