The fastest living organism is a bit of a tricky question to answer because "fastest" can mean different things depending on what you're measuring. Here are a few contenders, depending on the criteria:

Fastest in terms of speed:

* Cheetah: This terrestrial animal holds the record for the fastest land speed, reaching up to 70 mph in short bursts.

* Peregrine Falcon: This bird of prey is the fastest animal in a dive, reaching speeds of over 200 mph.

Fastest in terms of growth:

* Some bacteria: Certain bacteria can double their population in just a few minutes, making them the fastest growing organisms.

Fastest in terms of movement:

* Flagellated bacteria: These bacteria have whip-like tails called flagella that propel them through liquids. They can move incredibly fast, even faster than the fastest animals.

Fastest in terms of reaction time:

* Neurons: These nerve cells transmit information throughout the body at incredibly high speeds. While not a complete organism, neurons are essential for rapid reactions.

Ultimately, the "fastest" organism depends on how you define speed. There are many organisms that are incredibly fast in their own way, from the cheetah to the flagellated bacteria.