This is kind of opinion-based isn't it? I mean, I know this is Worldbuilding SE but this seems even more opinion based than usual since I assume you, personally, could easily think of any number of useful things like FTL drives, radiation shielding, anti-gravity generators, AIs that holds all the knowledge already, or an alien textbook on physics..

Antimater without means to convert it to energy of the form we can use - does not worth that much. All parts and stuff one can dig out - basically are an ore for reverse engenering, soo the best thing to find is a knowledge and technologies in a form of information, and the best case to have the means to implement them to be simple enough or/and laying nearby - tools to produce tools.

How highly organized artifact could it be? Could it be a Sci-Fi version of a Genie Lamp?

velcro - oh, wait ...

Antimatter is a consumable. Unless they stored millions of tons of the stuff, it'd be of limited utility. Much more important would be the machines to "manufacture" it. Maybe something that exploits an esoteric symmetry violation phenomenon. If no that, then a large-ish block of computronium, with a few billion yottabytes of advanced non-sapient software.

@DKNguyen Even with infinite possible answers, it can still be on topic as long as there is a criteria for what determines a best possible answer. Since the OP stipulates the goal "could advance the human race", and all technologies do this to different degrees, I think it can be assumed that the best possible answer is the one that does the most to advance the human race.

Gifting a large quantity of anti-matter seems incredibly dangerous especially if humans don't know what anti-matter is yet. They're liable to find a way to break into the canisters and kill everyone on earth before they figure out what they contain.

@Nosajimiki I don't know if it's possible to show an answer is best for "all of humanity". We are divided, there are many many viewpoints and cultures. Some might say post-scarcity is ideal, while others might say scarcity and conflict is vital to our being, for example.

@BMF "Advancing the human race" generally has less to do with what is best for individuals, and more to do with the availability of technology. Highly advanced civilizations are often depicted as dystopias. Being shitty places to live does not make them less advanced.

Unobtanium is the obvious answer. Comes with complementary Blue Avatars...

@Shufflepants wiping out all life on Earth with a single bomb is hard. It would take about 10 million kg of antimatter to equal the energy of the Chicxulub impactor... but humans are more adaptable than dinosaurs; so, you'd probably need at least hundreds of millions of kg to get the job done... A single "canister" might wipe out a large city, but the human race would likely survive anything small enough to get back to Earth.

@Nosajimiki A single tablespoon would wipe out a city. But if it's such small quantities, then it's not exactly useful to jump start an entire industrial civilization. Only enough to let one small project/one spaceship have the benefit if included is not a way to produce more. It's essentially no different from finding just a really compact battery. Could we do something cool with a single battery with the energy density of anti-matter? Sure, we could have one really fast probe or ship, but not much else without the ability to recharge or make more of them.

@Shufflepants If you are looking at antimatter the density of water, a tablespoon would yield a ~10km blast radius destroying most city centers but sparing thier metropolitan outskirts. A kg of antimatter would destroy most cities and thier metropolitan outskirts with ~50km radius. "Canister" is a vague term, but if we say you found something the mass of a barrel of oil, your explosion would still be limited to only about 250km... My point was just that mishandling a sample of antimatter would be very bad, but probably not end all life bad.

Thank you for all comments and answers ! I edited the question, be sure to check it out, I hope I clarified what consists of a best answer and what i mean by "advancement". English is not my native language, so I hope I didn't make many mistakes :) .

@Alexander Humanity could find a ship with very advanced propulsion system, pieces of machinery/weapons that ET's used (Laser cannons, light sabers...),pieces of their clothing or armor that had some advanced features, Alien Terraformers (or "Xenoformers")that would somehow alter the atmosphere, containers that contain rare gasses/elements..Im particulary interested in the last option(but not only).What exotic elements or particles could they have? Something that we could use to build new rockets, weapons,you name it.

@Nosajimiki Antimatter wouldn't go off like a nuke except in specially designed warheads. An antimatter bowling ball would just sit there glowing and hopping/sizzling on the floor... only the outermost shell of the antimatter comes into contact with regular matter, and it annihilates, but that tends to push away the regular matter which would be needed to annihilate with the rest of it. And given that you have little anti-carbons and anti-oxygens and all the rest, they don't annihilate very cleanly either.

@Mishima I am trying to name it :) One "genie" could be more than sufficient to replace all of that.

"Thousandstar" by Piers Anthony is about a competition between differetn worlds - a race - to see who gets to collect an alien artifact and profit from it.

I once watched a documentary about this topic... I think it was called "mass effect"