Even cute little baby Cell is still Cell. Where all of Dr. Gero's other creations were mechanical at least in some form, Cell was bio-engineered in a test tube from the cells of a veritable who's who of other Dragon Ball characters including Frieza, Goku, and Piccolo.
As the heroes stood in Dr.
Gero's lab and laid their eyes upon Androids 17 and 18 for the first time, little did they know that deep beneath them, in a hidden section of the facility, drifted a small creature in a large vat of liquid who, 17 years later, would emerge, kill Trunks, steal his time machine, and travel backwards to cause the Z fighters a whole mess of trouble.
This form rates bottom of the list for obvious reasons, and it was destroyed pretty quickly by Trunks and Krillin upon a second sweep of the area.