We have an explainer on our various ENSO indices here: 

https://www.climate.gov/news-features/blogs/enso/why-are-there-so-many-enso-indexes-instead-just-one

It is not the only region we monitor but we like the Nino-3.4 region b/c it is well correlated with other atmospheric measures of ENSO.  It is also one of the few regions of the tropical Pacific that is "Gaussian" which is handy from a statistics point of view (other regions are much more skewed).