Lem is excellent. GP, start with "Tales of Pirx the Pilot". Short stories with a great mix of interesting story lines, pragmatic view of the world and dry humor.
I like Lem's more serious books way more. "The Astronauts", his first SciFi is one of my favorites, because it starts out with a completely naive description of a happy communist world and gradually turns darker. It's an old book (and was probably written for adolescents), so the narrative style is pretty conventional, but I like how it causes chills in the second half.
Also "Solaris", although the descriptions of the various phenomenons on Solaris get a bit tedious over time.
And finally "The Invincible" if you like dystopian tales of artificial intelligence swarms going bad.
Of the serious ones "Fiasco" made a deep impression on me. The alien contact mystery is not bad, but the main theme is certainly human behavior and decision making.
