I'll recommend certain stuff that I consider pretty neat and that had undoubtedly influence some metal artist specially those on prog/power and neco-classical.
Antonio Vivaldi - "Opus 8, Il cimento dell'armonia e dell'inventione (The Contest between Harmony and Invention), 12 violin concertos including the celebrated work, Le quattro stagioni (The Four Seasons), consisting of the first four concertos in opus 8"
"The Mandolin Concerto in C major, RV 425"
Johann Sebastian Bach - "Toccata and Fugue in D minor, BWV 565"
George Frideric Handel - "The Water Music suites"
Ludwig van Bethoveen - "Piano Sonata No. 14 in C-sharp minor, 1st movement"
"Symphony No. 5 in C Minor (Op. 67)"
Niccolo Paganini - "24 caprices, for solo violin, Op.1"
Pyotr Ilyich Tchaikovsky - "Symphony No. 6 in B minor, Op. 74, Pathétique"
"Festival Overture: 1812, op. 49"
Edvard Grieg - "Suite Peer Gynt" (which includes "In The Hall Of The Mountain King")
Modest Mussorgsky - "Night on Bare Mountain"
"Suite Pictures At An Exhibition"
to mention a few examples