Final answer:
Mahler's peak achievement refers to the height of his compositional career, often associated with his Ninth Symphony or Symphony No. 10, though opinions vary.
Step-by-step explanation:
The term Mahler's peak achievement could refer to the height of his career as a composer, in which case his Ninth Symphony or the posthumously performed Symphony No. 10 could typically be seen as candidates for his finest work. Gustav Maher's compositions rank among the most emotionally powerful in classical music, demonstrating a profound ability to convey deep feelings and complex states of mind through orchestral music. However, there is no objective measure for such a peak; different scholars and enthusiasts might point to different works as his greatest achievements, depending on their personal preferences and the criteria they consider most important.