The obvious answer is, of course, Lord of the Rings. It is the standard by which many (rightly or wrongly) judge every epic fantasy since. If you are going to say you read fantasy, you need to read this.
For a lesser known work, I would say The Rose of the Prophet by Weiss and Hickman. I liked the setting- a nomadic, desert world, and the interaction between the world of men and the world of the gods. You also don't see a lot of fantasy with a more Arabic twist- there are djini and coniving spirits, oases and sandstorms. The three are an enjoyable read, imo.