Didn't think this was going to be difficult until I actually thought over it and realised how many series I love, but I'd have to go for:
The Discworld Series by Terry Pratchett - Recommended for anyone who likes fantasy. Or comedy. Or books. Or even just people in general. I'd even extend it to particularly literate animals. Generally anything with a respiratory system. To summarize, the Discworld is a magical world consisting of a flat, round disc of land balanced on the back of four elephants, who on turn stand on the shell of the Great Turtle A'Tuin. The series is well-written, funny, incredibly intelligent, and covers themes as wide as rock and roll, time travel, quantum physics, racism, religion and opera. To name a few examples. Not to mention the fact that Pratchett's incarnation of Death is one of my favourite literary characters.
The Hitchhiker's Guide to the Galaxy by Douglas Adams - Recommended to anybody who likes the Discworld series (and vice-versa), the completely random, yet brilliant, inventions and concepts, as well as the bizarrely twisted logic of the author, makes these books a must-read. If you don't find yourself laughing, I'm afraid you are deceased, and should seek immediate spiritual assistance in crossing over to the other side.
The Night Watch Trilogy by Sergei Lukyanenko - Very well plotted series, technically fantasy, but steeped in so much intrigue and plot twists that at times it almost reads like some kind of spy novel. Basically, the world is filled with Others, magical entities such as wizards, witches, werewolves, vampires and the like, who are governed by three main bodies; The Night Watch, a group of Light beings who enforce the law on the Dark Others, The Day Watch, a group of Dark Others who keep an eye on the Light Others, and the Inquisition, an organisation that tries to maintain balance between the other two groups. The story focuses on the various intrigues between these organisations, and is a fascinating and engaging read.