Others have recommended Mozart's Requiem if you like music with singing. If you haven't heard Robert Levin's completion, e.g. https://www.youtube.com/watch?v=457pvjOo9Hw, I think it takes the cake.

For a neophyte, I think it's good to start with the war horses, for which there are lots of high-quality modern recordings.

- Bach Brandenberg concerti (I like the Netherlands Bach Society, they have tons of great Bach recordings on Youtube. If you're into Bach, highly recommend you check them out.)

- Goldberg variations (I like Glen Gould's later recording, but Kimiko Ishizaka is also a great libre recording, and Netherlands Bach Society has a great recording on harpsichord)

- Mozart Symphony 40

- Mozart piano, violin sonatas

- Beethoven Symphonies 3,5,9 (I really like https://www.youtube.com/user/hrSinfonieorchester)

- Chopin waltzes, etudes, nocturnes

- Rach piano concerto 2, Rhapsody on a Theme by Paganini

- Sibelius violin concerto (I like https://www.youtube.com/watch?v=J0w0t4Qn6LY)

- Bruch violin concerto

Personally I think watching videos helps bring the music alive in a way that listening to recordings doesn't. And maybe it's heresy to some, but I think modern recordings are, in general, a lot more pleasant to listen to.

HTH!