If there was one thing, one piece of information that legally obligated all web hosting companies to reveal and make public - what do you think it should be?

The age of the owner perhaps?

Just one? I have to say their contact information. And the correct one. But shame on those that do not even make sure it is public in the first place before ordering from places that do not openly provide their address and contact phone numbers.

An Actual Domain WHOIS or Contact Information on their website not hidden or faked.

their company registration information, how many kiddie hosts/hosts that exist for a month are registered? It'd make identifying potentially dodgy hosts a lot easier

A big package of everything (one thing being the big package).

I am not sure I understand what that means...

The one thing: A big text file containing all the info I want. @ Artashes.

I think one is very very low. I would make a chain of information and I will ask them to put in their site for public viewing. Nowadays it has become so hard to reveal the truth. Who cares for the original information regarding this. I personally think a counsel has to be set for this(like ICANN). And I am sure this can prevent more illegal information.

Like CrystalVPS I would say it has to be company registration information. IT would separate out a lot of the legit ones from the kiddie ones.
But then again i know a hosting company that's ran by a kid aged 15 and they are doing quite well as far as i can see.

That assumes though that company registration is compulsory. I don't think all countries require that if you're a sole trader, with revenues under certain levels.