Ah, Fafhrd and the Gray Mouser! Leiber created some of the very best S&S creations ever with these two. So many fantasy writers, from M John Harrison to Terry Pratchet, owe a debt to Leiber, as do countless games, films, television series, and such. I cannot recommend them highly enough. The City of Lankhmar and the world of Nehwon are singularly unique creations, positively redefining the word 'exotic'. Smog-laden streets and curling alleys, inept thieves' guilds and adventuresome cutpurses, wings in the night swoop from mysterious towers to claim their female victims, strange shops appear from bare empty walls, eyeless and playful wizards seek the mask of Death, sentient castles and swamp houses on legs... And then of course there is the dirty duo themselves. No S&S writer, or fantasy writer period, has captured such witty and playful interaction between two fast and loyal friends. At their very best, the Swords tales attain an almost humming resonance between the two, playing both to perfection within the tale. Where Conan relied on brute strength and unwavering determination, where Elric relied on his sword and the help of dark gods, Fafhrd and the Mouser rely only on each other, a far more satisfying state of affairs.
As for my favorites, I've always loved the earlier series Swords Against Death. Though not as lyrically written as his later work, they contain such abounding imagination and excitement as to create an almost perfect fusion of the elements that produce great S&S. His later works, whilst enjoyable, sometimes played too much for humorous situations, almost becomming a parody of his earlier work.