Hard to say - I've read quite a few of his books, but not all of them (yet).
The one that got me started was a short story collection, Blind Willow, Sleeping Woman, and I just fell in love with his writing.
My favourite so far is The Wind-Up Bird Chronicle, but others are pretty great too, Norwegian Wood, 1Q84, Sputnik Sweetheart, and Men Without Women (another short story collection).
His work sometimes gets kind of weird and unreal and unsettling, but in a good way, kind of like David Lynch's best work, at least in terms of the feelings of unease it can provoke. Or, if you've ever seen Meshes of the Afternoon, that's got a similar eerie feel.
Hope you enjoy!