wanted to vote ... but the vote link leads to error 

My vote goes to 
-- Lord of the Rings Trilogy (Peter Jackson / 2001 - 2003) 
Reasons: 
- ALL 3 are consistently good ... in terms of quality and standard ... 
- Both critics and mainstream audience can appreciate the film 

Some comments about the rest 

- Three Colors Trilogy (Krzysztof Kieslowski / 1993 - 1994) 
Comments: Great, but really cater to artsy audience 

- Star Wars Trilogy [original] (George Lucas / 1977 - 1983) 
Comments: Kind of too long ago (i was only btw 8 to 12); 
On rewatching, the effects have been kind of "overtaken" by recent technology; 
As for the story, it remains "classic" but not as "gripping" in terms of pacing (maybe becos we know the story too well now) 

- Godfather Trilogy (Francis Ford Coppola / 1972 - 1990) 
Comments: The 3rd is quite a disaster 

- Infernal Affairs Trilogy (Wai-keung Lau, Siu Fai Mak / 2002 - 2003) 
Comments: The 1st was great, the 2nd was okay, the 3rd was kind of messy ... 

- Back to the Future Trilogy (Robert Zemeckis / 1985 - 1990) 
Comments: Too mainstream 

- Matrix 
Comments: The 1st was great, the 2nd and 3rd was kind of draggy, surviving only on effects 

Gonna catch up on the rest of the trilogy ...