TV tropes has this under big bad
tvtropes.org/pmwiki/pmwiki.php/WarriorCats/Tropes0ToGI would say Sol as big bad as Power of Three.
Tigerstar I guess under Omen of the Stars. Even though I think Brokenstar was the leader of the Dark Forest by the end, Tigerstar was the manipulator.
Big Bad: Tigerstar is generally considered the main antagonist of the series, but the arcs and some individual books have their own main villain.
The Original Series: Tigerstar
Into The Wild has Brokenstar by virtue of his leadership of ShadowClan and attempt to rule the forest.
Fire and Ice, Forest of Secrets, and Rising Storm split the role between Tigerstar, Brokenstar, and Nightstar, each of whom are major enemies of ThunderClan.
A Dangerous Path sets up Tigerstar as the true villain, but has the dog pack as the main antagonists.
The Darkest Hour has Tigerstar again, until Scourge kills him and takes over the campaign to rule the forest.
The New Prophecy: Appears to be Hawkfrost, but it's actually Tigerstar, albeit with Hawkfrost as his chief enforcer.
Moonrise: Sharptooth, the mountain lion terrorizing the Tribe.
Starlight: Mudclaw, due to his rebellion against Onewhisker.
Power of Three: Sol, the traveling cat who turns the Clans against each other.
Outcast: Stripes, the leader of the Tribe Invaders.
Eclipse: Other than the main villain of the arc, we have Onestar, who turns on ThunderClan and tries to destroy them.
Long Shadows: Ashfur, who becomes a major enemy of Lionblaze, Jayfeather, and Hollyleaf during his insane crusade to destroy Squirrelflight.
Sunrise: Hollyleaf, who is the murderer the Clans are trying to find.
Omen of the Stars: Either Tigerstar or Brokenstar, depending on who actually leads the Dark Forest.
The Fourth Apprentice: The beavers, whose dam is draining the lake and causing the Clans to struggle with thirst and starvation.
The Forgotten Warrior: Sol, back for one more attempt to destroy the Clans.
A Vision of Shadows: Darktail, for the first half of the series. For the latter half, there is no villain, and instead focuses on conflicts between the five Clans and the main characters.