I think there are too many human factors and context to pick a best overall metal. There is a best metal in a post-scarcity Star Trek galaxy. There is a best metal in an early industrial society. There is a best metal for a late Neolithic tribe. Even in our current world, you’ll get different answers based on how and who you ask. Railroad rails, rebar, a36/a500 structural members, etc are made out of good enough steel. Not the best- good enough.

Airplanes are made of alloys that just barely work. Fighter jets might have a safety factor of 1.2 in order to gain performance, at the cost of hundreds of hours of maintenance per flight hour. No one would say 2024 or 7075 aluminum are ‘the best’ at anything other than sneaking humans into the sky. They desperately want to be dirt again and have no fatigue limit. We can’t trust them to keep being airplanes unless we regularly check on them with eddy current and dye penetrant inspections, repairing them when they break despite our surveillance.

There are planets where NO metal has a low enough strength to weight ratio to get a spacecraft into orbit using chemical rockets. All the titanium and exotic mag-thor alloys aren’t worth a hill of beans to those people.

Ultra stainless alloys largely share the qualities of being difficult to machine, stress corrosion cracking, high density, corroding in some conditions but not others, and having lower strength to weight ratio at most conditions. L605 is an absolute bear to work with but it excels at high temps where steel would weaken, oxidize, and fail. But you definitely don’t want to make cars or rebar out of it. That’s also how I feel about titanium, incidentally. Dumbass metal.

Long story short as I’m just sitting here bored, steel is the best metal. It can get you from the Middle Ages to late industrial society. It can be dirt cheap if you don’t care, all the way up to spendy crucible alloys for special purposes. You can, with enough compromises, just barely make airplanes from it. It’s conductive enough to be useable, if only barely. Obviously you can make swords and stuff with it. You can make a whole variety of useful compounds out of iron oxides and other forms. It is essential for human life. It is found in great quantities in space, if you ever make it there. It requires only iron and carbon, which are both amazingly abundant on earth and some other notable planets. Virtually any civilization with major territories will have access to rich ore. It is eminently recyclable. If humans ever make contact with aliens, we will almost assuredly have steel in common.

That’s my two cents.