Deathstroke: Legacy - Part of the New 52 reboot, this is a more violent and exciting take on Deathstroke. Suggested for mature readers.
Deathstroke The Terminator - This volume consists of 60 issues, full of entertaining Deathstroke adventures. It's a good read by Marv Wolfman and Steve Erwin from the very early `90s, featuring Slade at his best.
Deathstroke The Terminator: Full Cycle - Issues #1-5 of Deathstroke The Terminator, also includes New Titans #70.
The New Teen Titans: The Judas Contract - One of the greatest Deathstroke stories, highly recommended.