[sgt101]

I pick Maori.

https://en.wikipedia.org/wiki/Treaty_of_Waitangi

More controversially I would pick India and China's encounters with Europe. I think it's possible to argue that both India and China were substantially more technologically advanced than the Europeans when significant links were established. India had maths and spinning (also probably better guns), China had silk, porcelain, paper, printing and the compass; also bigger better ships.

[nine_k]

Europeans reached India around 330 BC, in the form of the armies of Alexander the Great. I won't say that India folded in the result; it's rather the Alexander's makeshift empire folded.

There were several meetings of civilizations of comparable level of advancement and strength on Earth. I posit that in space it's significantly less probable, unless we consider these civilizations to also be sisters, all humanoids coming from a common source, like these of Earth.