What is the creature with the longest life span?

The animal with the longest known lifespan is a type of sponge that lives in the Antarctic Ocean: the antarctic sponge, with lifespan generally estimated to be around 1500 years. But you asked about "creatures" with longest lifespan, so I am wondering what you mean by creature. If you mean "living thing", then the list gets longer and more complicated. If you mean "individual", then the longest living individual organisms are found in the plant kingdom. It is especially easy to document longevity of individual trees, because they grow annual rings in their woody tissues. Yews, pines, figs, and olives have all been found to have individuals that are greater than 2000 years old, with an individual pine tree (Pinus longevae) in North America's Great Basin measured as being 5063 years old. Clonal plants can be estimated to being even older, up to 10,000 years old or more, and can be considered to be immortal because they continually produce young ramets (clonal plants connected to the same large genet, or genetically distinct individual). Unless a catastrophe occurs, or a huge shift in climate makes their current habitat uninhabitable, these plants could continue to grow forever. Clonal animals, such as the so-called immortal jellyfish, Turritopsis dohrnii, also live for incredibly long time spans. The immortal jellyfish cycles between adult medusa and larval hydra forms, and so unless it meets an untimely death, could live forever without aging.

By creature, if you meant "large enough to see with my naked eye", then I've covered many of the bases above. However, if you are willing to include microorganisms, there are some spores of bacteria that have been harvested from amber or coal and have been estimated to live for millions of years in their dessicated state. However, skeptics believe DNA would break down over that time frame, and believe the bacterial spores counted in the specimens could be from contaminants. The jury is still out on this one, but there does seem to be growing evidence that some bacterial spores can resist environmental degradation for a long time, especially if they are preserved inside something stable like amber, rock, or coal. Researchers are using extreme measures to ensure that their extraction equipment is sterilized to reduce the likelihood of contamination to a very small chance.