To be very unorky for a second the best god is Tzeentch. The shenanigans he and his followers pull alone makes him the best. Seriously he tricked Slaanesh and Khorne into fighting each other for the lulz. The other three are tied for second. Though i g0otta admit i have a soft spot for Slaanesh.
The best chaos god is the one purified by holy fire! These foul gods of the warp desire blood, then they shall have it in abundance at the end of my chainsword! HI-YAAAAAAAAAAA!
It is obvious, Tzeentch is the greatest of the Chaos Gods. He is the amalgamating force behind Chaos Undivided and crafty enough to make others think they are in control when it Tzeentch who is pulling the strings.
Oh sister you being so zealous to defeat chaos fuels chaos even more. No matter how you deny it you are a slave to darkness while chaos laughs and cackles gorging upon those you "purify".
Khorne (Kharneth) Is the Greatest of the Dark Gods for the following reasons! 1. He is the mightiest and the second oldest of the four major Chaos Gods 2. He is wrath incarnate, the embodiment of a never-ending lust to dominate and destroy 3. End of All things, a legendary blade capable of laying waste to the substance of worlds with a single blow, a single sweep can cut through reality itself, allowing Khorne's daemonic legions to spill forth into the Materium 4. His only temple is the battlefield, his sole sacrament the spilled blood of nations 5. Every single life taken in anger increases the Blood God's power 6. It is said that if Khorne himself should rouse from his throne and personally go to war against the other Dark Gods, his favoured blade would end them all in one mighty sweep, but that such an act would have calamitous results that not even Tzeentch could predict