Isotopes
The problem with novel isotopes are that we basically know all of the ones stable enough to potentially use in technology. And if there were one that were extremely rare, but useful, there is the question of whether a colony ship would contain enough of it to be useful for the entire human civilization. For instance, tritium is essential to the D-T fusion reaction, and very expensive for us to obtain. And while the fuel is very energy-dense, an ideal D-T fusion reactor could only get about 10 MW of power from 1 kg of DT fuel for one year. A 1 GW power plant would need about 100 kg of DT just to run for one year (not taking into account generation losses, inefficiencies, etc.). How many kg of tritium can you fit on a colony ship? Knowing about tritium isn't the problem. We can synthesize it from scratch if we really have to. Getting at economical reserves is the challenge.
The same problem applies to virtually every other raw material: if it's rare, then the amount on a colony ship would probably not go very far. Fundamental particles are even less useful, because we know pretty much all fundamental particles which can occur below absurdly high energies (on the order of TeV).
Molecules
Where aliens could really launch us forward is by introducing novel molecules. These may indeed teach us a new use for an isotope we already know about. High-temperature superconductors are the most obvious civilization-advancing example, but low-loss fiber optics for quantum internet, high-power magnets for electric motors, or new battery charge carriers could all lead to dramatic improvements in human technology. The advances would not come from knowing that strontium-88 is useful, or finding a few hundred kg on a colony ship. The advance would come from seeing it in a compound, using X-ray diffraction/crystallography, mass spectrometry, etc.
At this point, I hope you notice a theme: the most valuable resource is knowledge. In this case, the knowledge is obtained by reverse-engineering. It's more difficult, prone to error, and may fail or take a long time. But at the end of the day, knowledge is really the only thing that causes society to advance. Limiting the knowledge to an isotope is useless, because we already know all the isotopes (and their masses, reactivity, half-lives, relative abundance on earth, etc.). At the very least, open it up to molecules, and the world's your oyster.
Propulsion
A particularly useful molecule would be a catalyst/enzyme which enables a hydrocarbon battery. That is, having a way to oxidize hydrocarbons which converts all of the bond energy into moving electrons would make it possible to have electric airliners. They would be as fast/powerful as jet airliners, but would go twice as far and half as loud. All vehicles could be made hydrocarbon-electric (which still emits CO2, mind you, but none of the nitrous oxides and other combustion byproducts).
Energy
The most popular fusion technology is magnetic confinement (thermal) fusion, which is limited mainly by the difficulty of maintaining chaotic plasmas in a magnetic field. At best, a very powerful magnetic material would help advance the field, but we already know how to build very powerful magnets.
Where materials may help energy generation more would probably be on the fission side. There is currently a lot of work on molten salt reactors, and interest in thorium breeder reactors. Knowing which exact compounds form the best fission fuel and coolant could leapfrog our fission power generation capabilities, and thorium + nuclear waste is very abundant on earth. There is also the problem of radioactive embrittlement of containment structures (pipes, pumps, shielding, etc.) where new materials could make plants safer, longer lived, etc.
Weapons
The problem with weapons is that it covers a broad scale from individual combat up to interplanetary combat. The weapons at the ends of the scale have absolutely nothing in common, except for the need to concentrate sufficient energy on the target to overcome its defenses. A single material is not going to be useful across that range. Most likely, a colony ship is not going to be loaded with infantry weapons (they would send in marines for that). Nor should we expect it to be loaded with weapons you would find in their space navy (torpedoes or bombs). At best, it might have something equivalent to Star Trek's "navigational deflector", which is basically a space laser for shooting junk in your flight path. A colony ship is most likely not going to carry a store of hard ammunition for such a device, because that would be absurdly expensive. They must have access to very high energy power systems, so it would almost certainly be a laser of some sort.
Optically pumped lasers just run on electricity, so whatever advances occur there would most likely be in power production, wiring (superconductors), and cooling. The National Ignition Facility has some pretty big lasers, so there are no obvious impediments for future humans to build lasers as big as they like (besides cost and demand, of course). Chemical lasers, on the other hand, depend on certain chemical reactions to operate, and I'm sure we have not exhausted the space of potential chemicals.
Biology
Where aliens could bring the most radical changes to society would most likely be in nano-engineering. The problem is that reverse-engineering nano-scale structures is extremely difficult, and ones designed by aliens may be well beyond our abilities.
However, if the aliens have carbon-based biology, and we were able to decode this biology, then they likely also have a massive catalog of organic molecules which do all sorts of useful things (like the catalysts mentioned above). If their biology is at all like ours (cellular, encoding information in a biomolecule like DNA), then it should be possible for us to essentially "read" their technology by sequencing whatever biosamples they have on board. This is another way for humans to acquire technology without being given a handbook. The potential applications span the entire breadth of your question, easily.
Specificity
The question as posed is unanswerable, because it presumes that one of the users of WB are themselves an alien species as described. We can't name an isotope whose discovery would advance the human race because then we would have already exploited it! And if you are asking for a hypothetical such element, then any handwavium would do. I would suggest thinking of directions you want humanity to go in, and focus on those applications, rather than trying to invent alien technologies that we haven't seen yet.
