Hi there!
I'm an old fan of Stanislaw Lem, from mid 80s when i first read Solaris. Apparently i was too young at that time, so the real greatness of this novel came to me much later. Nonetheless, i liked it back then and reread it many times, maybe 6-8 times so far...
My favorites are 'hard' SF type ones: Solaris, Invisible, all of Pirks series, Magellan nebula (or how it is translated in English).
In my mind, Lem was a brilliant author, but the combination of the talent and a real fundamental knowledge in physics, cosmology .. general science is what made him the SF master... a rare mixture i guess.