Add to favoritesOnly going with issues Peter David wrote, or direct crossovers.
Volume 1 (31 issues)
Showcase ’96 #8 (final story)
Supergirl #1-6
Supergirl Plus #1
Supergirl #7-23
Resurrection Man #16
Supergirl #24
Resurrection Man #17
Supergirl #25-26
Supergirl #1,000,000
NOTE: PAD did not write Supergirl #13. Also, Supergirl #1,000,000 probably doesn’t tie into the main run so much, but it falls at a convenient place where it can be read as an extra. Resurrection Man #16 might be able to be skipped, especially if you already have a RM volume. Supergirl’s minimal involvement is recapped smoothly in Supergirl #24.
Volume 2 (26 issues)
Supergirl #27-35
Young Justice #12
Supergirl #36
Young Justice #13
Supergirl #37-50
Volume 3 (30 issues)
Supergirl #51-80
There were two annuals to this series, neither written by PAD. I don’t think the series got too involved during the company wide crossovers to where other issues are needed.
Shout out to jmbarnes at the Uncollected Editions forum for the volume breaks. I found his map after I had already put this together, but without volume breaks. He even included pictures which showed decent volume sizes.
You should definitely include Fallen Angel in this project. It is a direct sequel to this Supergirl run, also by Peter David. Obviously he had to be a bit coy about the DC comics IP, but there’s no reason it shouldn’t be considered a direct continuation.
Also, I believe Supergirl Plus actually takes place between Supergirl 5 and 6.