The forces of Chaos.
The Orks.
The Dark Eldar.
The Tyranids.
These are the 'bad guys' of the 41st millennium. They're also all bottom-tier codexes. The one codex that's both a villain and not bottom-tier is Necrons, who've been inching away from their villain status.
This is a bad thing.
One of the best-known theories of storytelling is that a hero is measured by the villains he faces. Superman would be far less impressive if he only fought common criminals. Green Lantern would be far less heroic if he focused on jaywalkers. Likewise, Space Marines don't look badass and heroic for beating up Chaos Marines any more. They look like bullies. Eldar don't look heroic for holding the line against tyranids any more, they look boring as their massed rapid fire hit and run rending and strength-D flamers leave wave after wave of dead bugs.
To be frank, this meta needs a better class of villain. Let's give it to them.