Tommy Monaghan–great character, or greatest character?! This series is aweseome from start to finish: zombie sharks, war stories, drinking with Green Lantern, time traveling dinosaurs, saving Batman’s butt, and ten thousand bullets (well, probably more than that, actually).
Hitman Volume 1
Demon Annual 2
Batman Chronicles 4 (“Hitman”)
Hitman 1–3 (“A Rage in Arkham”)
Hitman 4–7 (“Ten Thousand Bullets”)
Hitman 8 (“The Night the Lights Went Out”)
Hitman 9–12 (“Local Hero”)
Hitman 13–14 (“Zombie Night at the Gotham Aquarium”)
Hitman Annual 1
Hitman 15–20 (“Ace of Killers”)
Azrael 35 (“The Angel & The Hitman”)
Hitman Volume 2
Hitman 21 (“Kiss Me”)
Hitman 22 (“The Santa Contract”)
Resurrection Man 9–10
Hitman 23–28 (“Who Dares Wins”)
Hitman 29–33 (“Tommy’s Heroes”)
Hitman 1,000,000
Hitman 34 (“Of Thee I Sing”)
Superman 80-Page Giant 1 (“How To Be A Super-Hero!”)
Hitman 35–36 (“Katie”)
Hitman 37–38 (“Dead Man’s Land”)
Hitman Volume 3
Hitman 39–43 (“For Tomorrow”)
Hitman 44–46 (“Fresh Meat”)
Hitman 47–50 (“The Old Dog”)
Hitman/Lobo: That Stupid Bastich
Hitman 51–52 (“Super Guy”)
Hitman 53–60 (“Closing Time”)
JLA/Hitman 1–2
If you don’t want to include the Azrael or Resurrection Man issues, that’s fine, they’re not essential. These volumes aren’t bursting at the seams, though, and Tommy didn’t exactly have much facetime outside of his own series. Your call.
Hitman/Lobo was published the, but it fits into the story around issue 28 or so. Ringo is still alive, and the events that they reference place it around the African trip.
There is also a Hellblazer issue with a three page aside about the Ace of Winchesters which would serve as a nice intro to the Ace of Killers story.