Smoking. Millions of euros of taxpayer money spent every year on those lung cancer patients which could be well spent elsewhere. It's also an activity that negatively affects not just the smoker but everyone around them.
Qualified immunity for police officers. Prosecutors and judges basically get qualified immunity, too-- in that they can be caught engaging in all sorts of inappropriate and illegal activity without facing punishment because like police, it usually doesn't even get to the extent of being charged.
Requiring agreement to some unspecified ever-changing terms of service in order to use the product you just bought, especially when use of such products is required in the modern world. Google and Apple in particular are more or less able to trivially deny any non-technical person access to smartphones and many things associated with them like access to mobile banking. Microsoft is heading that way with Windows requiring MS accounts, too, though they're not completely there yet.
I think tacking on irrelevant laws onto popular bills to get them passed shouldn't be allowed.
Politicians shouldn't be allowed to trade stocks, especially when they're in a position to pass laws that would directly affect their holdings.
Super PACs, it's absolutely wild that that's a thing IMO.
I think there should also be a "cooling off" period of some sort over passing/repealing laws. I'm thinking as an example of the Republicans after Obamacare was passed, when they tried to repeal it something like 70 times in 10 years. I get that things change and laws sometimes need to be amended or updated, but there should really be some system in place to prevent people from spamming up the whole system like that.
Small print, excessive legalese and outright deceptive language in ads, agreements and such. All the "free" (not really free) trials, "unlimited" (not really unlimited) plans, "best value" (according to the producer and their mum) deals and shit like that.
There really should be a law prohibiting that - if reading through terms and conditions for using a damn website or a toothbrush or whatever requires 4 hours of free time, a magnifyibg glass and degree in law, such t&cs should be illegal. Same for disclaimers and such in ads - any 4pt text displayed for 2 seconds on screen should automatically result in a massive fine.
It violates the theory of inalienable rights that implied the abolition of constitutional autocracy, coverture marriage, and voluntary self-sale contracts.
Inalienable means something that can't be transferred even with consent. In case of labor, the workers are jointly de facto responsible for production, so by the usual norm that legal and de facto responsibility should match, they should get the legal responsibility i.e. the fruits of their labor
"By buying [unnecessary product] you will help [marginalized group] to gain a livable income and also send their kids to school instead of sending them to [work place with - even for adults - horrible work conditions]. Also, when buying [product] we will save [arbirtary area] of [rainforest/ coral reef/ mangrove swamp] that would otherwise have been destroyed [but not by us]. Additional to that, your purchase helped us to save [arbitrary ammount of CO2 - at least in a completely hypothetical scenario]. While using [product] you will make the world a better place."
As a customer there is barely any way you can ensure or check that these things are true. It cannot be possible to save the enviroment while buying stupid products like, for example, internet-of-shit-devices which will be phased out in no time or single use products made from plastic or other harmful materials that are not recycleable.
All these claims are just an indulgance trade - like it is done for centuries in a religious context. It is just that you have an excuse to consume more, because they to something to help people/ enviroment. If there was a product that would have been advertised as: "Well, we irretrieveably destroyed 100 km2 of nature, and for each single product in average two workers died and at end-of-life this product will fuck up the environment once more - also it will impair your health just by existing", it would be horrible - but at least it would be honest.
Honestly? Alcohol. I used to work security at a rehab, and it was always the worst addiction. The withdrawls are horrible, up to and including death. Yes, even worse than heroin.
Not really something that is legal that should be illegal, but I would love to see this nonsense that corporate executives can't be held criminally responsible (the corporation is) for their illegal acts.
I think it would correct an awful lot of shitty things corporations do really quickly (assuming enforcement).
Private cars in cities.
They're noisy, unhealthy, cause massive damage to infrastructure, transport one person at a time while taking up enough space for ~10 in the road, fill open spaces for parking, sometimes while being completely unusable, endanger everyone else on the roads....
Surprised to see no one has said cigarettes yet. Not only are you poisoning yourself, it's harmful to everyone else around you that has to inhale that shit.
Products purposefully manufactured to be unfixable. Main example being anything apple...you can't even add ram to a iMac anymore and they have the audacity to sell the lower end model with 8 gigs of ram as if that's enough when everyone knows its not. Basically selling a 1400$ piece of shit.
Everyone not having access to a 1-bedroom apartment or living space that is all theirs and affordable. So much crime is because people are forced to live with others they shouldn't be around and can't get along with in a shared living space.
Additionally, so many people are driven by the fear of homelessness so they just suck it up to their detriment until they snap and go really nuts and end up with shelter either way
Lying if you're a politician. You should be in a state similar to "under oath" in court, but at all times.
Advertising. I should have the legal right to not be advertised at. I should have the right to not have to accept advertising in order to access services, especially so if I already pay a subscription to that service. I cannot put into words how much I loath and despise advertising and advertisers. I hate them. Hate, in the real sense of the word.
Loot boxes in video games, whatever age group the game in question is aimed at, but especially so in kids' games.
Microtransactions in video games for anything other than non-essential/non-advantageous items, like cosmetics. Even then, their presence should upgrade the PEGI rating to adult/18, regardless of the actual content of the game. This might help prevent their inclusion at all.
Whatever the fuck is going on in Gaza right now.
Shielding police or soldiers from prosecution for crimes they've committed. If you stand in the way of the due process any other citizen would face, you should be heavily penalised for that. Like, the murderer soldiers who carried out Bloody Sunday are currently being protected by the British state and its lackeys, and I cannot fathom why. Cops in the US who murder people are frequently protected by unions and get to retire rather than be fired etc. All of that shit should be illegal. Get the fuck out of the way of due process.
Naziism and related nonsense like Holocaust denial. Germany already has laws about this, but that shit needs to be legally smothered in its crib everywhere.
Conspiracism surrounding public health issues like vaccines and masks.
non-consensual advertising (consensual being things like steam discovery queue, where I actively want to be advertised to), "lobbying" (bribing), fossil fuels and friends, gerrymandering (US), the electoral college (US), publically trading your company
Possibly controversial but prostitution. Allows for regulation and workplace safety. Would probably calm a lot of men down as well and help them focus on the more important aspects of getting into a relationship.
Edit: misread the title and thought it said "legal".
I feel like my answer might break AskLemmy's rule 2 about "Overt Politics", but so do a lot of the other answers? Feel free to delete if so.
overtly political answer, also CW for violence.
As far as the current American system goes... nothing. By and large, even laws that seem good are mostly only used in service of the elites, against the people. Consider this series of events:
In 2015 a white supremacist in South Carolina commits a mass shooting, killing 9 people.
In 2017, the Georgia state gov expands the state's domestic terrorism laws, directly in response to this shooting, because the previous version wouldn't have covered it.
In 2022, this expanded law gets used... against people protesting police brutality, who hurt no one, despite the fact that the cops killed one of them.
Unfortunately, this general sequence is not uncommon at all. Neither is the inverse, where the bureaucrats/judges/etc decide "that doesn't count, actually" when it comes to an elite very clearly breaking an existing law, or else changing the law so it doesn't apply to them in retrospect.
From my industry: Perhaps the purchase of chemicals for the manufacture of fireworks. It's surprisingly easy to order pounds and pounds of different oxidizers and fuels.