Protista

I would assume Insecta is the largest, unless you count Protista which actually seems to be a ragbag of totally unrelated things that biologists cannot not be bothered to classify properly.

Where are all the malacologists? AYDIN 2009

The taxonomic kingdom Protista is a collection of single celled organisms that do not fit into any other category.

Protozoa 2008

July 25, 2006, 8: 30 am online texas hold strategy card free says: online texas hold strategy cardfree rectangles pages, Pakistani raiding Protista

The Volokh Conspiracy » Arafat. Godfather of Terror: 2004

Foraminifera, called forams by those who study them, are mostly microscopic and exclusively marine organisms in the kingdom Protista.

Archive 2006-11-01 AYDIN 2006

Foraminifera, called forams by those who study them, are mostly microscopic and exclusively marine organisms in the kingdom Protista.

A shell of a different kind AYDIN 2006

“Okay, the five classes would be, from most advanced to most primitive, Animalia, Plantae, Fungi, Protista… and Eugene.”

Night World No. 2 L.J. SMITH 1997

The first half of this "Progonotaxis hominis," which has no support from fossil evidence, comprises three groups: (i) Protista

Evolution in Modern Thought Gustav Schwalbe 1880

Infusoria (Ciliata, Acineta, &c.), and many other Protista, remain throughout life one single cell, and, nevertheless, this cell is as fully furnished with all the most important attributes of the soul, with sensation and volition, as any one of the higher animals with a nervous system.

Freie wissenschaft und freie lehr. English Ernst Heinrich Philipp August Haeckel 1876

Haeckel introduced the new apperceptive name of Protista, which ended the disputes.

Talks To Teachers On Psychology; And To Students On Some Of Life's Ideals William James 1876

Sixty years ago such a doctrine was excusable, for then nothing was accurately known either of the finer structure of the brain, or of the physiological functions of its separate parts; its elementary organs, the microscopic ganglion-cells, were almost unknown, as was also the cell-soul of the Protista; very imperfect ideas were held as to ontogenetic development, and as to phylogenetic there were none at all.

Monism as Connecting Religion and Science A Man of Science Ernst Heinrich Philipp August Haeckel 1876