Exalted Sorcerers
"Twisted arcanists, wicked of intent and strange of form, Exalted Sorcerers have an encyclopædic knowledge of spells and are able to bend reality to their desires. Amongst their number are warrior-mystics that have stalked the galaxy for ten thousand years – to these masters of the esoteric, matters of war are a crass distraction, but these visionaries bring a terrible focus to bear when battle rages. Enemies are torn to shreds by devastating magical barrages, hideously changed, twisted and distorted as the most ill-fated Chaos Spawn." ~Games Workshop
Exalted Sorcerers are some may consider as a step-up from your conventional Chaos Sorcerer . In this case, Exalted Sorcerers are often handpicked by either Magnus the Red or Tzeentch as their most favored servants. Whilst not on the same level as a Daemon Prince, Exalted Sorcerers possess an intricate knowledge of both the Warp and Sorceries, allowing them to channel certain magical spells to create powerful weapons of war.
Most Exalted Sorcerers are given the privileges to ride upon a Disc of Tzeentch where they are given the flight advantage to shoot out doom bolts and Troll whoever is unfortunate enough to face them. Expect a ton of Just As Planned when these guys are around.
7th Edition
Exalted Sorcerers are one of the most cheesy units available for the Chaos Space Marines, let alone the Thousand Sons. Whilst a bit overcosted for what they are worth, their stats in the end, is worth it. All Exalted Sorcerers have a Mark of Tzeentch, is ML2 base, and can upgrade to ML3. They are BS5, I5, and W3 compared to a standard sorcerer. Also has access to a S9 AP2 Heavy 1, Lance Blast once per game thanks to his Lord of the Silver Tower special rule. Also, he's Fearless! Gets access to Divination too, which the Sorcerer doesn't know unless you give a Crimson Slaughter Sorcerer the Balestar.
Combine these bastards with Ahriman and its G-fucking-G. Enjoy drinking the tears of butthurt players as you spam a mindbogglingly amount of mind-bullets in their general direction. Very little can survive this onslaught on the first turn. So dial-up the Troll Meter and engage in unsolicited psychic rapestorm.
8th Edition
The psychic phase having been either completely gutted or finally brought back to tolerable levels (depending on who you ask) in 8th edition, none of the above still apply. As of the Chaos Index, an Exalted Sorcerer is a bit of a jack-of-all-trades character. Slightly better in combat than a regular Sorcerer and with a command aura to boost Thousand Sons units; and can manifest two psychic powers/deny one each turn. A bit underwhelming right now, however if the new CSM Codex is anything to judge by the Codex: Thousand Sons will give them some extra goodies to go Just as Planned on your opponent with.