If a landing spacecraft accidentally ignited the methane oceans on Titan, and the event was caught on camera by the orbiter.... that would be a "wow" moment for me.

Aside from that, if any rover, sent to any planetary body in the solar system, found anything interesting, quite unlike what we've repeatedly seen on Mars, it might be a "wow" moment. But only for a moment.

Wow moments could come from biology - if someone found a way for people to live indefinitely using relatively inexpensive periodic medical treatments. That would be totally unexpected. For me.

Genetics - if someone found a way to re-engineer, synthsize the genomes of, extinct animals. Jurassic park.

Physics - if someone found a way to manifest force at a distance without an exchange of material. A reactionless drive that teleports momentum to other objects from a distance. That would be awesome.

Biological electronics - a mass of neurons in a tissue scaffold that performs the functions of a simple electronic device, like an FM radio, or a calculator. WOW.