The Invincible is my favourite book, period. I just love the theme of humanity bringing its most powerful, most advanced weaponry against an "enemy", and discovering not that it can't defeat it, but that doing so is simply....pointless.

I've just finished reading Lem's Hospital of the Transfiguration - I love that this theme of pointlessness is expressed even in his non sci-fi books, it's just absolutely stellar reading.