Glory to the Dark Gods!!!
Chaos for me, in particularly the warriors of Chaos though the interaction between them, the beastmen and daemons is pretty interesting IMHO. I love how despite the power they have attained, they must still always fight for the favour of their Gods, challenging all, even other Chaos to prove their might. They also have a strange code of honour, not all good and noble like the empire or HE but fighting fair to prove their strength without deception or betrayal (except devotees of Tzeentch).
Also, their influence affects all other races, from corrupting the empire to Chaos energy creating the Skaven to being the entire reason for the creation of the lizardmen. They are strong enough to take on any and every other army and win due to the allure of Chaos leading to betrayal etc and the power given to them by the Gods of Chaos.
The different personalities of Chaos characters following different Gods are interesting as well. Khorne trying to prove their strength and glorify their God on the battlefield. Tzeentch trying to attain all knowledge and increase their power. Nurgle trying to show the whole world the futility of resisting Nurgle's foul embrace, revelling in all diseases and pestilence. Slaanesh trying to receive ever greater depraved pleasures, always wanting more and more as they receive their grandest dreams. And regular undivided guys like Wulfrik, cursed to challenge the greatest warriors the world has to offer to prove that he is the ultimate fighter. All trying to attain the ultimate goal of immortality and daemonhood.