Hmmm. It depends partly on what kind of stuff you like.
Lord of the Rings is basically an adult continuation of The Hobbit, and is a specific narrative of a period of time over a few years -- the war that ended the Third Age, with the resolution of the war with Sauron and the One Ring... all the Rings, as a matter of fact... and of the elves. It zooms the camera in pretty close as a personal narrative, compared to the Similarillion which takes a more "bird's eye" view of events.
The Silmarillion is more historical and mythic in how it's written. It starts with the creation of the world, where Morgoth/Melkor came from (he wove dark music against the communal music of Eru Ilúvatar's other children, attempting to insert melodies of his own against the grand design.. and thus he fell).
The name itself comes from The Silmarils, which were three (?) gems of exceeding beauty that captured the light of the Two Trees in Valinor (= heaven) which at some point were sucked dry and killed by Ungoliant, Shelob's mother, with Morgoth's help -- these gems were created by Feanor, one of the high elf craftsman of the time, but basically became the center of a long war due to everyone wanting them. Many of the other stories of middle-earth history revolved around the silmarils, the attempts to steal them, curses that were attached to them, etc. It is a tale of tragedy and woe, pride and destruction. So you are basically tracking important narratives/myths across large expanses of time, seeing how the Silmarils impacted history.
Some of the most important stories are there -- the fall of Morgoth and the rise of Sauron, his lieutenant. The story of Beren and Luthien, which the later relationship between Aragorn and Arwen reflects. The story of the exodus of the elves. The tale of Turin Turumbar. Lots and lots of tragedy in that book, it can be depressing if you don't like tragic stories. I love tragic stories, and sometimes it really hurts to read those stories... especially the Huor and Hurin debacle. But really, Tolkien considered it his main work on Middle Earth.