Floch isn't just the best villain in Attack on Titan - he's also the only real antagonist Hajime Isayama's anime ever introduces. Debuting in Attack on Titan season 3's "Bystander," Floch Forster began as a fresh-faced Survey Corps recruit, wet behind the ears and greener than grass, despite training in the same 104th cadet corps as Eren Jaeger. When Eren later forms the Jaegerists and launches a coup on Paradis Island in Attack on Titan season 4, Floch is only too happy to lead the charge. With Eren busy using the Founding Titan to stomp Marley into a parking lot, Floch effectively finds himself in control - a role he openly relishes, plumbing one moral depth after another.
The true villain of Attack on Titan is, of course, Eren Jaeger himself. Obsessed with protecting his friends at any cost, Eren triggers the Rumbling, resolving himself to destroy the entire world if it ensures Mikasa and the others can live freely. Eren is the genocidal monster Eldia and Marley forge an unlikely alliance to combat, the story's most powerful single character, save the mighty Ymir Fritz, and unquestionably Attack on Titan's final villain. Yet Eren is hardly a traditional antagonist. For starters, he spends the entirety of Attack on Titan seasons 1-3 as the story's main hero. The signs were always there, sure, but Eren's villainy only crept out in season 4. Also, Eren's actions - while horrifically misguided and violent - are rooted within a desire to protect the people he loves.
Similar caveats apply for other villains we've met throughout Attack on Titan. Kenny Ackerman was a brutal enemy and a worthy foe, but his backstory and tough upbringing made him too sympathetic to be considered truly villainous. More hateful than Kenny was his employer, Rod Reiss. Alas, Rod was bound by the Fritz family's vow of renouncement - and also only appeared in a small handful of episodes. King Fritz and Sergeant Major Gross (the guard who shot Grisha's sister) are both utterly irredeemable, but appear even less than Rod Reiss, ruling them out of contention for a main villain berth also.
That makes Floch Forster the only true main villain in Attack on Titan. Absolutely nothing in Floch's backstory makes him even remotely sympathetic. Whereas most Jaegerists act out of reluctant self-preservation, Floch is motivated by an obvious love for the power Eren grants him, and a fondness for the Jaegerists' "us or them" ideology. Floch takes a sick, perverse pleasure in killing Eren's enemies, acting with way more violence than any reasonable person would deem necessary, and that trait becomes even harder to stomach when considering how - in a fair fight - any of the main Survey Corps characters could absolutely destroy him. In truth, Eren must've identified these darker qualities in Floch's personality and exploited them to ensure his Jaegerists remained in control (and the Rumbling wouldn't be prevented).
A lack of redeeming features, an unhinged personality, and a smug grin that only a mother could love combine to make Floch the only genuine villain of significance throughout all four seasons of Attack on Titan. And that's no bad thing, as Hajime Isayama builds Floch's devilish nature from his very first appearance. Attack on Titan season 3 introduces Floch as someone who transferred to the Survey Corps from another division, immediately demonstrating his distaste for actual combat and proper hardship. Floch riled up audiences further by showing cowardice during season 3's Shiganshina battle, questioning Erwin's orders, then having the audacity to survive while braver men perished. As if Floch hadn't already inspired hatred, he then opposed Eren and Mikasa reviving Armin. The absolute swine.
Floch expressed his displeasure at the Scouts' ceremony - a bold move considering his limited contribution in battle - and then refused to hold back against civilians during the Battle of Liberio. These events all happened before Floch even became an official Attack on Titan villain, but each nagging remark and display of ruthlessness marked him as someone to watch, laying the foundations for his eventual crimes as a Jaegerist. Even if there were other remorseless, long-term villains in Attack on Titan, Floch's cockiness toward superior officers (Keith Shaddis especially), his cruelty toward anyone deemed even vaguely anti-Eren, and the smile with which he goes about such grim business would be enough to earn the title of Attack on Titan's greatest "proper" villain.
Attack on Titan season 4 continues Sundays on Funimation, Crunchyroll and Hulu.
