Oh well, that's a HAAAARD question:) I'll tell you this way: my own personal favourite is Fiasco (Polish: "Fiasko"), but this book is not very popular. BUT I don't think it is the BEST of Lem's book objectively. I know Lem himself always said that his own favourite, and the one he considered his best, was Cyberiad ("Cyberiada"), or maybe Solaris.
Cyberiad is indeed a work of genius - it is imaginative, and VERY FUNNY. So if you like funny books, read it. Solaris on the other hand, is very serious - phylosophically deep, emotional. It's not an easy book. Fiasco is actually sad:)
Of course there are also very popular Tales of Pirx the Pilot which are entertaining, quite serious, and have romantical sense of adventure.
But if I had to tell you only ONE book of Lem, I would definitely say Solaris or Cyberiad.
Regards!