Here is the complete series with all crossovers and extras.
Volume 1
Underworld Unleashed 1
Aquaman 14
Azrael 10
Damage 18
Deathstroke 53
Green Arrow 102
Guy Gardner: Warrior 36
Manhunter 12
Extreme Justice 10
Flash 107
Primal Force 13
Spectre 35
Underworld Unleashed: Apokolips – Dark Uprising 1
Green Lantern 68
Hawkman 26
Justice League America 105
R.E.B.E.L.S. ’95 13
Starman 13
Detective Comics 691
Fate 13
Impulse 8
Ray 18
Steel 21
Volume 2
Underworld Unleashed 2
Catwoman 27
Damage 19
Green Arrow 103
Guy Gardner: Warrior 37
Lobo 22
New Gods 2
Robin 23
Underworld Unleashed: Batman – Devil’s Asylum 1
Batman 525
Extreme Justice 11
Legion of Super-Heroes 75
Primal Force 14
Spectre 36
Showcase ’95 12
Superboy 22
Underworld Unleashed: Abyss – Hell’s Sentinel 1
Green Lantern 069
Hawkman 27
Justice League America 106
Underworld Unleashed: Patterns of Fear 1
R.E.B.E.L.S. ’95 14
Adventures of Superman 530
Detective Comics 692
Justice League Task Force 30
Underworld Unleashed 03
Legionnaires 32
Ray 19
Fate 14
Superman: The Man of Tomorrow 3
Robin 24
Aquaman 18
Starman 71
Just making sure, these are the issues in the series. Many of the issues work together like detective issues 691 and 692 for example. Also, the story is rather non linear and some say that Azrael 10 should be first.