Which of the following is not aromatic?

Aromatic compounds must satisfy Huckel's rule, which states that a planar, cyclic, conjugated system must have $(4n + 2)\pi$ electrons to be aromatic, where $n$ is an integer.

Cyclopentadienyl anion has $6\pi$ electrons ($4$ from two double bonds and $2$ from the lone pair on the carbon atom), which satisfies $4n + 2 = 6$ for $n = 1$. Thus, it is aromatic.

Pyridine is a heterocyclic aromatic compound with $6\pi$ electrons ($6$ from the ring atoms), satisfying Huckel's rule. Thus, it is aromatic.

Cyclopropenyl cation has $2\pi$ electrons from the one double bond, which satisfies $4n + 2 = 2$ for $n = 0$. Thus, it is aromatic.

Cyclooctatetraene has $8\pi$ electrons. According to Huckel's rule, systems with $4n$ electrons are anti-aromatic if planar. However, cyclooctatetraene adopts a non-planar tub-shaped conformation to avoid anti-aromaticity, making it non-aromatic.

Answer: Cyclooctatetraene