It's hard to say which you will find to be the best example, but I'll share a couple of the ones that mean a lot to me.
I'll preface this by saying that almost any behavior can be written off as something besides intelligence, but at the same time, we have no reason to believe it's not intelligence.
1) Inter-species cooperation. Groupers, eels, and octopi have all been seen hunting in pairs with one another, usually with the same individual each time. This cooperation is usually brought on by the animals making gestures rather than simply appearing and hunting the same fish. Not all groupers or moray eels engage in this behavior.
Barracuda have also been known to dwell near docks in Hawaii until fisherman show up, where they then direct the fisherman towards the nearest school of fish so they can get some food.
2) We've seen several taxa of fish show Machiavellian intelligence, which is essentially the ability to behave diplomatically or to use information one knows about another individual to their own benefit.
https://www.researchgate.net/publication/292242763_Machiavellian_Intelligence_in_Fishes
3) Research on optical illusions that had only previously worked on primates and humans work on several taxa of fish. These optical illusions play on advanced cognitive understanding of a prior experience.
4) Many taxa of fish use multiple brain areas at a time, something associated with higher levels of cognition. Individual fish have also been observed to have left and right brain bias.
5) Zebrafish and goldfish trained using color and reward puzzles have shown metacognitive abilities. AKA the fish was aware when it didn't know the answer. They become hesitant and choose not to answer. They can think about thinking. I got to see this in person since we were the ones doing this assay.
6) Zebrafish have complex brain processes and social habits that become obstructed in the exact same way by the same neurodivergencies. In other words, autistic fish behave in the same way as autistic humans. Fish with Alzheimer's show the same symptoms as humans with Alzheimer's (mostly). Zebrafish have proven to be an even better model for autism then mice have.
These are just some of the examples. We also see tool use, jealously, extremely good memory application and puzzle solving abilities in fish, to name a few.
Take them as you will.