The Ju-on Collection

The Ju-on franchise began life in 1998 as two short films directed by Takashi Shimizu that formed part of a TV Movie named Gakkô no kaidan G (1998); they were Katasumi (In a Corner) and 4444444444 (Ten Fours), respectively.
It then moved to V-Cinema for The Curse I + II (2000, both) before finally making the leap to a full theatrical release with Ju-on: The Grudge (2002).
Its success was such that Hollywood soon followed with a series of English language remakes, two of which were even helmed by Dir. Shimizu himself.
If you only want to experience the best that each region has to offer, the Nut Blogs contributors recommend number 03 in the Japanese language selection below, and number 02 in the English language selection:

Japanese Language Films:
01. Ju-on: The Curse (2000)
02. Ju-on: The Curse 2 (2000)
03. Ju-on: The Grudge (2002)
04. Ju-On: The Grudge 2 (2003)
05. Ju-on: Shiroi Rojo (2009)
06. Ju-on: Kuroi Shojo (2009)
07. Ju-on: The Beginning of the End (2014)
08. Ju-on: The Final Curse (2015)
09. Sadako vs. Kayako (2016)

English Language Films:
01. The Grudge (2004)
02. The Grudge: Director's Cut (2004)
03. The Grudge 2 (2006)
04. The Grudge 3 (2009)

NOTE: Sadako vs. Kayako (2016) is a crossover with the Ringu franchise. For short reviews of films in that series see the Ringu Collection.