Simply put, a bait that has the most attraction qualities. The fish need to know that your bait is present and that the bait will not harm them. Fish are curious, cautious and suspicious by nature. Fish will gather around a bait waiting for the first brave fish to take a bite. As soon as the first fish successfully gets a piece of the bait the other fish are less cautious. So if you are bottom fishing, sometimes you may have to place segments of bait on a single hook to get more strikes without losing your whole bait.
Another thing to keep in mind is the type of fishing you are doing. Is it freshwater or saltwater? Saltwater bait is more productive for saltwater fishing and freshwater bait is more productive for freshwater fishing. If possible, give the fish something they're use to eating. If you keep fish to eat and you clean your own, you can cut open the stomach and examine the contents to see what they are eating at the time. You're likely to find partially digested bugs, worms, minnows, frogs and shellfish. This will provide a hint about what to use the next time you go fishing.