The SR-71. Titaniumn accounted for 93% of it's structural weight. At the speeds it flew, the sustained high heat soaking of the SR-71 took airframe temperatures to 570 degrees C. At that temperature, aluminum would practically melt, or at best, have the strenght and consistency of soft licorice.
Aluminum is light and cheap, but loses it's strength with elevated temperatures. Steel and titanium can withstand heat a lot better before elevating temperatures cause loss of strength. Aged B-120 titanium weights only half as much as stainless steel per cubic inch, but has a similar tensile strength.
But titanium is more expensive, and more difficult to fabricate. It also has a few weird flaws that require special handling. Titanium is not compatable with chlorine, florine, or cadmium.