Kind of depends on what you want. /u/ThisManNeedsMe nailed it with the best incarnation of the character but the problem is that isn’t what the current incarnation is. The New 52 changed the character and what is in Injustice is closer to the Pre New 52 version. So if you want to pick up the books featuring Deathstroke’s ongoing exploits it will be with a different character but if you just want to read good Deathstroke stories, continuity be damned, then there are lot of good stories. Unforchunatly most of those stories are out of print because DC doesn’t want you reading anything that contradicts their current marketing for the characters. So you’ll have to find some way to read them despite that.

• Deathstroke The Terminator: Full Cycle
  
• Nightwing: Renaegade This one is cool considering the history the two characters have.
  
• The new Teen Titans: Terra Incognito & The New Teen Titans: The Judas Contract is the legendary story about Deathstroke and why he is such an important character.
  
• Identity Crisis isn’t about Deathstroke but he plays a decent role in it. It’s a damn good book and it has one of the coolest Deathstroke fights ever put to paper.
  

Modern DC Character Origins

• Aquaman | Time and Tide - David | The Trench - Johns
  
• Batgirl | Year One - Dixon, Beatty
  
• Batman | Year One - Miller | Zero Year - Snyder
  
• Deathstroke | The Judas Contract - Wolfman | Deathstroke, The Terminator: Full Cycle
  
• Flash (Barry Allen) | Rebirth - Johns | The Flash - Manapul, Buccellato
  
• Flash (Wally West) | Born to Run - Waid
  
• Green Arrow | Year One - Diggle
  
• Green Lantern | Secret Origin - Johns | DC: New Frontier - Cooke
  
• Hellblazer | Saga of the Swamp Thing #37-44 - Moore | Hellblazer #1-13 - Delano
  
• Joker | The Killing Joke - Moore | The Man Who Laughs - Brubaker | The Joker - Azzarello
  
• Nightwing | Year One
  
• Red Hood | Under the Red Hood
  
• Robin (Dick Grayson) | Year One - Dixon, Beatty
  
• Shazam | The Power of Shazam! - Ordway | The Monster Society of Evil - Smith | Justice League back-ups #0, 7-11, 14-16, 18-21 - Johns
  
• Superman | The Man of Steel - Byrne | Birthright - Waid
  
• Swamp Thing | Saga of the Swamp Thing - Moore
  
• Teen Titans | The New Teen Titans v1 - Wolfman | Teen Titans v1 - Johns | Year One - Wolfram
  
• Wonder Woman | The Legend of Wonder Woman - De Liz | The True Amazon - Thompson | Wonder Woman - Azzarello | The Lies/Year One (Rebirth) - Rucka
  
  Suggestions to improve this list are welcome.
  
  EDIT: Added Shazam! suggestion
  

I would be shocked if he did, judas contract, would be the best place to start. Plus a ton of stuff came out of that arc.

EDIT: looks like I spoke to soon
http://www.amazon.com/Deathstroke-Terminator-Cycle-Marv-Wolfman/dp/093028982X
part of a 65 issue run