Warhammer 40K's lore is riddled with warfare among factions. Which of the many sides of the conflict are the most powerful?

The Imperium is a big place and sometimes Planetary Governors and others decide they'd rather go solo. That's when Rebellions happen.

This massive daemon and tiny angry woman might be the most shipped pair of war criminals in the whole expanded Warhammer ...

Games Workshop is planning a big push for the new Chaos codexes. What does this mean for Chaos and the rest of 40k?