Hilarious Fantasy - Terry Pratchett's Discworld series (there's like 20 books now). No, you don't have to read them in order. Inspired two PC Adventure games.
Funny Fantasy - Robert Asprin's Myth series. He quit writing them around 1992, but he just released a new one and claims he's working on the next one.
Serious Fantasy - Fred Saberhagen's Sword Series (The First Book of Swords, the Second Book of Swords, The Third...well, you get it)
SF - Fred Saberhagen's Berserker books. Not really a series, just a common theme.
Isaac Asimov's Robot Novels (The Caves of Steel, The Naked Sun, The Robots of Dawn, etc)
"Historical" SF - David Drake's Belisarius series. What If...the Roman Empire acquired Gunpowder?
And...this one doesn't really fit Fantasy or SF, but how do you classify a series about an assassin that outruns cars, kicks people's heads off, runs up the side of buildings, and keeps fighting a robot that just won't die? Anyway, it's durn amusing: Warren Murphy & Richard Sapir's
Destroyer Series. There's like 128 books now. No, you don't have to read them in order. They're very light reading, and I own most of them now. Inspired a movie and a TV pilot.
Does anyone know if the "new" Foundation series (after Asimov died) is worth reading?