While there are many volcanic eruptions throughout history, there's no specific mountain eruption that stands out as the most significant in 1886.

To help you find the information you're looking for, could you tell me:

* What region or country are you interested in?

* Are you looking for a specific type of eruption (e.g., explosive, effusive)?

* Are you interested in the impact of the eruption, such as casualties or environmental changes?

Once I have more details, I can provide you with a more accurate and helpful answer.