Orchesta

In no particular order:

Beethoven: Any of the symphonies. I would recommend starting with #5 (which everybody pretty much already knows, but ought to listen to anyway), then #7, then #1, #3, maybe #4 and 6. Beethoven's 9th Symphony is one of the greatest pieces ever written, but is also less approachable for the beginning listener.

Mozart: Symphonies #40 and 41.

Rimsky-Korsakov: Scheherazade

Stravinsky: Firebird and Petroushka ballets

Orff: Carmina Burana

Tschaikovsky: Symphonies #4, 5, 6; Nutcracker Ballet; Romeo and Juliet

Copland: Appalachian Spring, Fanfare for the Common Man, Rodeo

Prokofiev: Lieutenant Kije Suite

Bizet: Carmen Suites

Dvorak: New World Symphony

Adams: Short Ride in a Fast Machine

Haydn: Surprise Symphony

Bach: Brandenburg Concertos (any of the six)

Vivaldi: Four Seasons

Brahms: Symphony #1, Tragic Overture

Strauss, Richard: Til Eulenspeigel