Generally, the less dense the material is, the stronger the buoyant force will be. The strength of the buoyant force may not be the only thing you may be concerned with, however.
A substance which has a very low density and which is not too hard to manufacture is hydrogen gas. If you fill a balloon (or dirigible) with it, it will float in air. Hydrogen has the undesirable property of being flammable, explosive even, and large collections of it make for large explosions, as evidenced by the Hindenberg accident.
You can put helium in balloons and blimps and they will float just fine. Helium leaks out of rubber very slowly. Blimps aren't very practical forms of transportation, but they work fine as advertisements and things to float over sports arenas.
Hot air is less dense than colder air, and you can fill hot-air balloons with this material and they will float too. Air can be heated with a flame and it's cheaper to use this than to buy a large amount of helium for a short balloon trip. Sometimes cost and simplicity of storage and distribution is the overriding factor, not just the buoyancy.
Substances lighter than air are usually gases (I cannot think of any liquid or solid
that's lighter than air at the moment. Possibly an aerogel with helium in it might work..)
But then you need some kind of container to hold the gas.
Did you mean floating on water? Or oil, perhaps? Styrofoam works pretty well for floating on water. It isn't very rigid, and it melts at a low temperature. It also decomposes if you leave it out in the sun for a long time. If you're making speedboats, I'd not recommend making them solely out of styrofoam. They'd float great, but they'd fall apart too easily.
You can make combinations of things that have some components heavier than water (or air) that will float if they have large volumes of less-dense material inside of them. For example, a steel-hulled boat will float if it has enough air space inside. Steel is good for structural strength and low cost. Most large ships are made of steel, although fiberglass is often used for small pleasure craft and sailboats. You can also use just about any kind of wood, for ease of assembly and low cost, but it gets waterlogged, rots and isn't as strong as steel.
(published on 09/08/06)