I use all three but care needs to be taken as all three can have errors and place songs on the wrong artists or combine artists. BMI generally combines all works for a artist under 1 “artist name” query, and if you look at each work shown that work will show the name and IPI assigned to the work.
BMI also allows (or use to allow) the rights holder (or rights manager) to do strange “things”, the law firm representing C.J. (Carl Jones) Records has combined many of the works under a single IPI for CJ (that is a 2nd IPI) so that when you query that name you get a listing of all the works under that 2nd iPI (different artists). Just be careful when using any of them.
ISWC pulls information dynamically from different databases depending on your country locality, that is why you see multiple entries with the same iswc string. I have had information from ISWC that had not been updated and was wrong. Just recently I had issues around father and son having the same name getting combined together.
In all cases you need to remember that some (maybe a lot) of the work information may still be on paper and not added to the databases, and that information is by “name” not IPI so BMI/ASCAP researcher needs to find the correct IPI or even create a new on. I have found over a dozen works placed on the wrong artist name because they were “almost” the same name. I was talking to a artist and he told me that they would write the song name along with the writers names on a piece of paper and turn it in.
All I am saying is these databases are not the final word, you may still need to do more research and think through what you are being shown. For me I usually work an artist at a time and get to know the artist so I can tell when something is wrong.
EDIT: more thoughts
When using ISWCnet be careful of the "CA" composer/author designation after the Creator-Id, almost all works have it and that includes works that have no lyrics (composer only works).
Use as a reference not a fact. While factually accurate in most cases these are "Rights Holder" databases, think of the following cases as you are adding works.
A work is assigned to someone who bought it, but did not write it.
A work in the Public Domain (PD) or a traditional work (TRAD) has been reworked, who gets credit?
An artist reworked composition and or lyrics based on another authors work and gets a new IPI for the work.