Sitka spruce has been the most common top wood for guitars mainly because it is plentiful and very strong. Being a stiff wood it has a bright sound but has to be played fairly aggressively to bring out its volume. It typically sounds best when played with a flat pick and is the preferred wood for bluegrass.
Englemann spruce is softer and not as stiff as sitka spruce so less effort is required to get a good sound out of it making Englemann a good wood for finger style guitar. It has a light, even creamy color.
Adirondack or red spruce was the preferred wood used on pre-war Martins and has become popular once again. It is exceptionally stiff and strong but has a subtlety to it that allows it to work well with both flat picking and fingerstyle. Cosmetically it usually has brownish streaks and irregular grain but this does not deter from its sound.
Cedar is a very light wood, not as strong as spruce and has its own unique sound. A cedar top is mellow sounding emphasizing the mid range over the brighter trebles. It responds well to a light touch making it an ideal wood for fingerstyle playing. For studio recording and playing live with a pick up, cedar avoids the overly bright sound you can get from spruce.
Port Orford cedar is a rare and unusually strong cedar that only grows on Port Orford Island off the coast of Washington. Once used in the building of Japanese temples it is now used for guitar tops and has a sound in between that of western red cedar and the spruces.