Best bike frame material?

Both materials are excellent and practical. While both can be made into light frames, aluminum tends to be lighter. That being said there are plenty of cheapo alum. bikes sold at discount stores that are very heavy. So it's not just the material but the design and quality of the frame that makes a big difference.

Chromoly hands down. After time aluminum starts to wear down, and break. Aluminum is lighter, but it comes at a disadvantage. I prefer not to buy aluminum things for my bike, if they go under stress for any period of time they are permanently damaged, and break easily.