Trilogies

Ancient Greek Tragedy Authors wrote trilogies to illustrate the developing of a moral or existential drama through generations or simply through time. Aeschylus wrote (among many others, mostly lost to us) the Oresteian Trilogy, Sophocles the Oedipaean Trilogy. Trilogies were wiewed by Aristotheles to be a dramatc syllogism: Thesis,the posing of the problem or the breach of order, Antithesis,the consequences of the problem or wrong ways to restore order, Synthesis, which brought about the Catharsis or solution of the drama or moral dilemma.
Beyond Greek times, the Divine Comedy by Dante may be called a Trilogy, and so other literary artworks. But it's in contemporary Science Fiction and Fantasy that trilogies (and often Quadrilogies) are returned as a literary custom.
Trilogies are more compact than sagas, and often the novels can't stand alone by themselves. The first novel introduces, the second developes, the third concludes. That's the case of the Lord of The Rings Trilogy by J.R.R. Tolkien, a literary masterpiece and a curse on Fantasy genre. Why so many Fantasy Authors felt obliged to imitate and clone Tolkien? Why all those Dark Lords and zany companies of elves, dwarves and kitchen boys defeating them on the brink of disaster?
Science fiction Trilogies are more original, and many are really masterpieces of the genre. My favourite trilogies( or Quadrilogies)are:
-The Night's Dawn Trilogy of Peter F. Hamilton. A galaxy-spanning triloy, with various human and alien civilizations, fighting the evils of religious fanaticism of a satanic sects whilst a plague, unwittingly brought about by an alien being, menaces to destroy civilization.
-The Otherland Quadrilogy by Tad Williams. The Ultimate Virtual reality Saga. A sombre sect developed the Ultimate Vutual Reality whwre it's impossible to escape, at the cost of committing heinous crimes against humanity. A group of corageous websurfers manages to explore Otherland and defeat the monstrous minions of the sect, traveling from vitrual environment to another, some beautiful, some terrifying.
The Neanderthal Parallax Trilogy by Robert J Sawyer in a parallel Universe, Neanderthals developed intelligence and civilization, whilst ""homo sapiens" equivalenta, or Glikskins, died out. The two Universes, by quantum experiments being made, come in conctact...
There are so many others! Yhe Yilanè Trilogy by Harry Harrison, where intelligent reptiles and humans vie for supremacy; The Hitch-Hiker's Guide to the Galaxy Trilogy, by Douglas Adams, with its surreal humour; the Demon Princes Pentalogy by Jack Vance, where the survivor of terrible atrocity hunts down the Arch- Criminals that perpetrated it.
As you see, trilogies become quadrilogies, pentalogies, and then sagas. In fact, many sagas are made up of trilogies connected by novels, sometimes.
What's your favourite?