The compound or ion which is not aromatic in the following is

Those compounds which are having cyclic conjugation, planar and contain $\left(4\mathrm{n}+2\right)\mathrm{\pi }$electrons are aromatic compounds. The compounds which are having cyclic conjugation, planar and contain $4\mathrm{n\pi }$ electrons are antiaromatic compounds. 

, , are aromatic compounds. Their $\mathrm{\pi }$ orbital system has p electrons that are delocalized all throughout the ring. Also, they have $4\mathrm{n}+2$ delocalized p electrons.

 is an antiaromatic compound. Its $\mathrm{\pi }$ orbital system has p electrons that are delocalized all throughout the ring. But it has $4\mathrm{n\pi }$ electrons only.