We'll leave this question for the scientists to debate. What you need to decide is which one is best for you.
Each material has specific characteristics and outstanding bikes are built out of all of them. So what really matters is finding a bicycle that fits, rides and handles the way you like and one that suits your needs and budget. What the frame material is should almost be an afterthought.
Still, the buzz about steel, aluminum, carbon and titanium may leave you wondering if it isn't worth it to go after one or the other. To help, we list the features of the different frame materials below. You'll see that they're strikingly similar in some ways and that they're all ideal for use in bicycles. This means that you'll get a great bike regardless of what material you pick. (If you narrow your new-bike choices down to a few different models and can't decide, test ride both and pick the one that feels best!)
Steel: Classic look, lively ride, durable and easily repaired, fairly light, affordable, can rust if abused.
Aluminum: Modern look, lively ride, durable, corrosion resistant, lightweight, affordable.
Carbon: High-tech look, lively ride, durable, corrosion free, lightweight, usually a little more expensive than steel and aluminum.
Titanium: Various looks (depending on finish), lively ride, durable, corrosion free, lightweight, usually the most expensive material.