Big Fatty Chaos Ogres. These guys were a key element when the army first started, but they were shelved after the umpteenth battle in which they were shot to pieces before reaching the enemy. Now that these guys can get Chaos Armor and Mark of Khorne, they've come back to the tribe and have thus far kicked some serious butt.
Albion's Nobs. Empire White Wolf knights with marauder bitz. These five guys (Chaos Knights) have been the hammer of this force since the very beginning. One classic game was a 2250pt battle versus Skaven in which the knights began on my extreme left flank, killed a unit of plague monks, some censer bearers and globadiers, a few ratling guns, a unit of slaves and another unit plague monks. They ened up on my extreme right flank at the end of 6 turns, and had collected 3 standards, only losing 2 knights in the process (and thus retaining full VPs). Brutal!
Rickety Chariot. Orc chariot pulled by Empire light cav horses manned by a Marauder driver and Mordheim crewman. Empire White Wolf knights with marauder bitz. The chariot is pretty hit or miss in battle, but it's usually good for one or two turns of combat before fleeing off the table. They just like to mimic the drivers of Tampa, hit-and-run fatality capital of America.