Final answer:
Modest Mussorgsky is identified as the most nationalistic member of the 'Mighty Five' with a unique style demonstrated in his opera 'Boris Gudonov', despite having less formal compositional technique.
Step-by-step explanation:
The most talented and nationalistic member of the 'Mighty Five', known for a folk-like style and less equipped in compositional technique, particularly evident in his opera 'Boris Gudonov', is Modest Mussorgsky. Mussorgsky's work is known for its highly original use of blocks of sonorities.
However, he is often considered to have had a less formal mastery of composition compared to other composers of his time. His opera 'Boris Gudonov' exemplifies his unique style that was influential in developing a distinctly Russian voice in classical music.