Radio 3. And some would add Classic FM for introductions to famous repertoire.
Classic FM is definitely the place to start as they play excerpts from lighter, more popular pieces although this can be a bit misleading with some pieces as it is usually the best bits they play. Radio 3 is more serious stuff & could as easily put you off as attract you.
You have to get your mind around the fact that pieces can vary from 15 minutes to 4 hours (for some operas). Also the variety of the many different types is mind boggling. I have listed a few pieces which I would recommend you to try. They are the copies I have got but plenty of other versions to choose from.
Berlioz - Symphonie Fantastique - Royal Concertgebouw c Colin Davis - 55m
Beethoven - Symphonies nos. 5 & 6 - Berlin Philharmonic c Hebert von Karajan - 1hr 4m
Dvorak - Symphony no.9 "From the New World" - Baltimore Symphony Orchestra c. Marin Alsop - 1hr 5m
Handel - Water Music, Music for the Royal Fireworks & other - Academy of Ancient Music c Christopher Hogwood - 2 disc CD set
Holst - The Planets - London Symphony Orchestra c Colin Davis - 50m
Orff - Carmina Burana - Bournemouth Symphony Orchestra c Marin Alsop - 1hr approx. - I call this the "Druid Song" - something a bit different.
Mussorgsky - Pictures at an Exhibition - original is a piano piece (very rare) but Ravel orchestrated version is much more popular with wide choice
Rimsky-Korsakov - Sherherazade & other pieces - Mariinsky Theatre Orchestra (Kirov) c Valery Gergiev - I hr 2m
Tchaikovsky - 1812 Overture - often with March Slave, Capriccio Italien & others
Tchaikovsky - The Nutcracker - Mariinsky Theatre Orchestra c Valery Gergiev - This is a ballet but music is a favourite of mine - 1hr 20m
Verdi - Requiem Mass - Philharmonia Orchestra & Chorus c Carlo Maria Giulini - 2 disc CD set with others
Vivaldi - Four Seasons - Europa Galante c Fabio Biondi - 2 disc CD set with others - but also extremely wide choice of other versions
I am sure you will have heard excerpts from some of these although you may not have been aware at the time what they were from. They are in alphabetical order of composer but they are all very listenable - in my opinion.