LOTR, definitely! LOTR is epic! Bourne's trilogy is very good also.
Ocean's trilogy - imo only first part was great, but the others were bullshit. (( The same about Matrix (first part was great, but other parts not so good).
I never thought of it like that. There are great movies all around, but selecting the best trilogy...
One man's treasure is another man's trash, you know... Tastes differ, so different people vote for different things.
I would say I liked the 1989 -1992-1995 Batman trilogy most. With genuous Jack Nicholson as Joker, with Danny de Vito as Penguin and Tommy Lee Jones as Two-Face.
Because actors ACTED and there were REAL FIGHTING and REAL ACTION and REAL THRILL.
All the movies filmed on the green background, with all visual effects and part of the actors being only rendered pictures... they lack that sense of REAL ART.
I would even vote for Godfather, if GF3 was not so bad. GF2 is iconic, a masterpiece and owns a bundle of Oscars for a reason.
LOTR is awesome, because they managed to put some real soul into the picture. A legend became alive, a bundle of Oscars, etc. But there were so many movie sins, ScreenJunkies channel on Youtube lost count.
All series are best for someone.
I will vote for the first Batman trilogy (1989-1992-1995)