The cheetah is the world’s fastest land animal, going from 0 to 70 mph (113 km per hour) in just 3 seconds. Even a Porsche 997 can’t accelerate that fast. Nevertheless, the cheetah is the fastest only in terms of absolute speed but they can’t maintain that pace for long. Pronghorn Antelope, the second fastest land animal and many other antelope species can easily outrun a cheetah over a long distance. The cheetah must catch an antelope in the first minute or less of the chase; after that the cheetah has to slow down but the antelope does not (there is sayings that fear have long legs).

(Cheetah)

Fastest land animals

1) Cheetah 70 mph (113 km per hour)
2) Pronghorn Antelope 61mph (98 km per hour)
3) Wildebeest 50 mph (80 km per hour)
4) Lion 50 mph (80 km per hour)
5) Thomson’s Gazelle 50 mph (80 km per hour)

Fastest water animals

1) Sailfish (Istiophorus platypterus) 68 mph (109 km per hour)
2) Swordfish (Xiphias gladius) 60 mph (97 km per hour)
3) Marlin (Makaira) 50 mph (80 km per hour)
4) Wahoo (Acanthocybium solandri) (47.88 mph 75.6 km per hour)
5) Yellowfin Tuna (Thunnus albacares) leaping 46.35 mph (74. 6 km per hour)

(Sailfish, image credits: Rob Hughes)

Fastest air animals

Swifts can fly at 106 miles per hour (171 km per hour).

Peregrine falcons are averaging 25-34 mph (40-55 km/h) in traveling flight, reaching speeds up to 69 mph (112 km/h) in direct pursuit of prey, but can dive even faster – at impressive 242 mph (389 km/h) speed and easily catch the swift.

In other words, the short answer to the question “who is the fastest animal in the world?” is: peregrine falcons.

Human speed

The top speed of humans was clocked at 27.1 mph (43.56 km/h) by Olympic sprinters Michael Johnson and Donovan Bailey during their Olympic competitions. This top speed only lasts for very short bursts.
Exactly what makes one kind of animal faster than another is complex and not entirely understood. One surprise: Apparently, it doesn’t matter how many legs you have. Four-footed creatures are not inherently faster. Another surprise: All human runners move their legs at about the same rate, according to a study Weyand and his colleagues published in The Journal of Applied Physiology in 2000. “The slowest woman in our study, the time she took to reposition her leg was the same as Donovan Bailey, even though she could only sprint half as fast,” Weyand says. What matters is not so much how fast the legs can move, but how much force they can exert on the ground. One of the most important determinants of that is how much muscle can be fit on a beast’s legs.

Cheetah Analysis

Mako Shark The fastest Fish In the Sea Impressive (actually not the fastest but an example of high speeds in water)

The fastest animal on planet – Peregrine falcon

Sources:
http://www.wisegeek.com/what-are-the-fastest-animals-in-the-world.htm
http://wiki.answers.com/Q/What_is_the_fastest_water_animal
http://www.homeworkspot.com/ask/fastestanimals.htm
http://wiki.answers.com/Q/What_is_the_fastest_land_animal
http://www.forbes.com/2004/05/14/cx_mh_0514running.html
http://www.birds.cornell.edu/AllAboutBirds/BirdGuide/Peregrine_Falcon_dtl.html