Cat's Eye is far and away her best work. Really, really the best. I've read a lot of other things, and Handmaid's Tale comes close, but the more sci-fi-ish stuff is kind of meh, and some of the early fiction just isn't that great.
But Cat's Eye. If you like Margaret Atwood, read Cat's Eye.
I like them all, but I'm partial to Oryx and Crake. I think about it every time I pass a McDonald's (chickie nubbins). I also really loved Alias Grace and The Blind Assassain. Has anyone read The Penelopiad yet?
I think that Handmaid's Tale should be required reading for life. Everyone, everywhere. I love it and find it utterly terrifying all at the same time.