I'm not gonna take the time to read all the posts.

All materials have pluses and minuses.

Aluminum: Because of the lower density, you can have a very large diameter down and top tube, without the wall thickness being very dent-prone. Torsional rigidity increase as a 4th power (IIRC) of the diameter so this makes for a very torsionally rigid frame, great for people that hammer the pedals (racers) and touring bikes with heavy loads so the frame doesn't feel like a wet noodle. However aluminum fatigues (cracks) when highly stressed for long periods in reversing loads. So it has to be built rigid enough to resist that, so it rides more poorly. I bought an '89 Cannondale criterium frame with a 2" down tube, looked very racy in '89, but over time I put larger tires on it for a better ride. In retrospect, a less racy frame would have been better for me.

Steel: Good ride, but less rigid for the same weight. Steel can actually have higher strength to weight ratio than aluminum, but rigidity is not a function of strength, but rather elastic stiffness (young's modulus), which stays constant even as strength increases within a family of materials (i.e., high strength steel, it's still steel). You can get higher rigidity by going larger diameter, but to keep it light, then the wall thickness gets so thin it becomes easy to dent, and even not, subject to local buckling (wrinkling), you look at an old B-52 with thin skins and you'll see the same effect in the fuselage fore and aft of the wings.

Titanium: Very good fatigue strength, so can be designed to flex like steel, but has lower density so can exploit some of the same design advantages of aluminum but without the drawbacks. Expensive, welding takes special equipment. I currently favor titanium if I can find one cheap used.

Carbon fiber composite (carbon fibers in a plastic matrix): Can change the fiber direction in parts of the frame to tailor strength and stiffness. This should produce the most optimum ride and stiffness and lightest weight. And even prices of carbon frames have come way down. BUT, carbon frames are fragile and difficult-to-impossible to repair, and fail "catastrophically", i.e., breaks in two, rather than "plastic" deformation like most metals (ironic term) which is inherently safer in a crash. There are some respected bike shops that recommend against carbon anything for consumer bikes that are not raced.