[Tomassci (OP)]

What is the biggest bacteria/animal/plant/fungi cell? Candidates post under.

[evan_au]

The largest cells found in nature are ostrich eggs. Before fertilization, the entire interior of the egg consists of a single cell, which can weigh up to 5 pounds. The largest single-celled organism is an animal called Syringammina fragilissima, which can grow to a width of 4 inches.

See: https://www.quora.com/What-is-the-name-of-the-largest-cell

I imagine that some of the ancient megafauna with even larger eggs would have been even larger single cells, eg the elephant bird, with eggs up to 10kg and 34cm long.
See: https://en.wikipedia.org/wiki/Elephant_bird

[Kryptid]

Evan_au is right about the eggs, but some other interesting cases are algae such as Acetabularia and Caulerpa. They are so large, you'd never know that you were looking at single cells.