I just recently purchased these and haven't read through them yet, but here is the prospective reading order based solely on release dates. If there are changes that I discover as I read through them, I'll edit them. Otherwise, as always, if you see things that need to be added, changed, removed - let me know!
Gen 13 (Vol 2, 1995, Image) #33 - Preview story
Planetary/The Authority: Ruling the World One-shot
Planetary/JLA: Terra Occulta One-shot
Planetary/Batman: Night on Earth One-shot