LOTR of course. And I can’t add much: kate.lin named them all, with the possible exception of the Matrix.
But it’s not as good as the Dead trilogy. Not that it will remain a trilogy forever, unfortunately. Not sure if the last film will be as good as the first.
The early Star Wars films were indeed great but it’s not a trilogy: more like an heptalogy.
Mad Max
Road Warrior
Mad Max Beyond the Thunderdome
Samurai Trilogy, starring Toshiro Mifune
Miyamoto Musashi
Zoku Miyamoto Musashi: Ichijôji no kettô
Miyamoto Musashi kanketsuhen: kettô Ganryûjima
Great flicks.
[quote=“urodacus”]LOTR of course. And I can’t add much: kate.lin named them all, with the possible exception of the Matrix.
But it’s not as good as the Dead trilogy. Not that it will remain a trilogy forever, unfortunately. Not sure if the last film will be as good as the first.
The early Star Wars films were indeed great but it’s not a trilogy: more like an heptalogy.[/quote]
My list is getting piled up on my desk, not quiet finish yet! 
I am Taiwanese, I should introduce some good Taiwanese films.
Here is Ang Lee’ Father trilogy:
Pushing Hands
The Wedding Banquet
Eat Drink Man Woman
But Ang Lee’s movies always ruin my dates.
Everytime I go to see his movies, I always can’t help crying which always scares my date. 
HK trilogies too.
Infernal Affair I, II, III
Chinese Ghost story I, II, III
[quote=“Nicya”]Mad Max
Road Warrior
Mad Max Beyond the Thunderdome[/quote]
I didn’t mention these because, well Thunderdome blew goats.
Agreed, in fact for my mind that was a classic example of why you should quit when you are ahead.
Mind you, the first really big mistake was putting out a dubbed version of Mad Max with American accents. I amost fell off my chair when I heard some of those characters speak Amerrikun. It was almost as if for the Oz audience that picked the broadest Aussie accents available, and grunting such sweet local vernacular, and then decided to hide them from the world.
HG
Yeah–Beyond the Thunderdome was pretty bad. Especially the haircuts. Tina Turner already had some scary hair, but Mel gibson didn’t have to. Big hair of the 80’s I guess. (The band ‘Poison’ comes to mind.)
Still love Toecutter and the Humungus.
I am the Nightrider!!!
being the hater condemned to hell that i am, i like The Godfather I, II, and III, though III sucks.