Silmarillion was always his main work - he started writing it in the trenches of the first world war and continued it 'til his death. It was the Lord of the Rings that was based upon this history of Middle-Earth, planned and constructed far more thoroughly than any of his various other works. His son Christopher did excellent work piecing it all together from various notes, essays and other writings. However, Tolkien himself had done huge amounts of work during those decades and what ended up on Silmarillion is just a small reflection of what it could have been, had he finished it in time. Nevertheless, Silmarillion is to me the most important book I have ever read - and most probably will always be.
-Villain