In which type of rock would you find fossils? Why?

Of the three major types of rock formations (igneous, sedimentary, and metamorphic), sedimentary rocks are most likely to contain fossils.
Fossils are formed when the bodies or remnants of living organisms (plants and animals) are covered and are thus protected from predators or microbial decay. The ideal location for finding the fossils would be a rock formation where the conditions are such that the fossils are not exposed to extreme conditions.
Igneous rocks are formed when the hot magma cools down. Any plant or animal remains that fall into hot magma would most likely be destroyed due to high temperatures. Similarly, metamorphic rocks are formed when an existing rock is subjected to high temperature and pressure. Such conditions are also not conducive to the preservation of plant or animal remains.   
Sedimentary rocks, unlike igneous and metamorphic rocks, are formed by the gradual deposition and cementation of the material over time. Such rocks provide ideal conditions for fossils because plant and animal remains can be covered by layers of materials over time, without destroying them. 
Hence, fossils are most commonly found in sedimentary rocks.