One of the professors of my collage, who was a British, was a very fluent speaker of Japanese. Although his accent sounded like British certainly, his vocabulary and skillfulness of making a speech were obviously superior to those of the average Japanese people. From what I see, that proficiency was partly due to his openness and motivation for acculturating to Japan. He is also married to a Japanese woman, so he must have had plenty of opportunity to learn it.
To add one more person, one of my American friends is a brilliant language learner too. Compared to the professor I stated above, she still has much difficulty in grammatical aspect. But her speaking competence was exceptionally superb, considering her experience of learning Japanese was just one year when we first met. I find her feeling little anxiety in making mistakes or stumbling during conversation. And she also has a positive attitude toward Japanese culture.
Who is[are] the most advanced learner[s] of your language you have ever met?
To what extent s/he is proficient, and how is s/he like?
I've encountered two people who happened to be absolutely fluent in English. I couldn't believe it once I knew that they are non-native English speakers. One of them is from Iran, and the other one is from Europe (I forgot his country name). I think they can compete with natives themselves with their current level in English.
Edit summary: Fixed some mistakes
@drasvi, Thank you!!
I found the second girl, from Argentina, especially interesting. I suppose she must have learned some foreign language other than Russian. Otherwise, people would rarely make such an attempt, right? But if not, that's stunning indeed.