A Match-Up that will be debated til the end of time...
Undoubtedly the two most hardcore assassins Marvel and DC have ever seen, and virtually identical in almost every way. Deadpool's creator, Rob Liefeld, basically modeled the the character after Deathstroke. Writer Fabien Nicieza then even gave Deadpool the real name of "Wade Wilson," as an inside joke that he was related to "Slade Wilson."
But I will attempt, somehow, to narrow it down to one victor. One man left standing, or at least left breathing. I'll break it down into categories and assign 1 point to the man who takes that category. Those categories:
Deadpool: Not much is known of Wade's early career, or whether or not he received any special training. But he is a master martial artist and extraordinary hand-to-hand combatant, proficient in multiple unarmed combat techniques. He is no doubt one of the most skilled fighters in the entire Marvel Universe. Wade is also a master of assassination techniques, espionage, covert ops, infiltration and escape artistry, as well as a supreme marksman.
Deathstroke: A former soldier in the U.S. military with special ops training, becoming an army legend. An extremely advanced hand-to-hand combatant, being highly skilled in Boxing, Karate, Jujitsu and Ninjutsu. Also, a demolitions expert and proficient with many types of explosives, as well as an expert marksman in his own right. He's also a skilled hunter, something he likes to do when he's not out murdering people. Slade is also very adept at analyzing and understanding a situation, not many can catch him off guard, ever.
Based on Slade's military background, he gets the point.
Deadpool: Spent most of his career as a freelance mercenary, assassin or body-guard; working for criminals such as Hammerhead, Kingpin & The Wizard. While employed by The Wizard, he spent time working alongside Taskmaster and Constrictor, a short-lived version of the Frightful Four. He also spent time working for Department K, a special weapons branch of the Canadian government. Wade even became a S.H.I.E.L.D. Bounty Hunter during the Civil War, and also went on several personal missions for Dr. Strange. Over the years, Deadpool has faced off against the fierce X-Man Wolverine numerous times, even capturing him during one of their encounters. He's also defeated Bullseye, Taskmaster, Tiger Shark & even the entire Thunderbolts team single-handedly.
Deathstroke: A world-renowned, meta-human assassin. Once a soldier in the U.S. army, having fought all over the world in places such as Bosnia, North Korea, Southeast Asia and Russia. He is a master strategist and can take control of virtually any situation. During a mission in Moscow, he faces down 10 armed Russian soldiers, and within mere moments, slays all 10 men where they stand. One time, when he was annoyed, Slade flicked a paperclip through the air and sliced a fly in half, now that's insane. On a mission to take out an arms dealer, Slade jumps out of a plane, with no parachute mind you, and lands onto another plane, digging his sword into it to hold on. He then rips the door off the plane, which is moving at 500 mph, using his might alone. He then easily defeats a group of Clayface clones by throwing them out of the plane. Slade then jumped out of the plane as well, and grappled his way back into the original plane. He then betrays and kills his entire team, the Alpha Dogs. Defeats multiple villains that went by the name Legacy. There was a time when Slade's youngest son Joseph Wilson/Jericho is holding his other son Grant hostage, threatening to stab himself and Grant with a sword, without hesitation Deathstroke drives the sword into both of them, killing them instantly. That's pretty cold blooded.
Due to Deadpool's many failures over the years, and Deathstroke's high success rate, Slade gets the point.
Deadpool: Main power is his insane healing factor, even greater than Wolverine's. He can heal from virtually any wound, even beheading, and regrow limbs and organs. Wade was even able to recover after The Hulk LIQUIDATED everything from his waist up. He will also use his healing factor as a tactical advantage in battle. His body is resistant to virtually all foreign chemicals and diseases. Virtually Immortal, cursed by Thanos to literally be unable to die. Also has telepathic immunity, but not necessary in a fight against Deathstroke. His strength is roughly 2 tons (4,000 lbs). Thanks to the Weapon X program, Wade also has superhuman stamina, being able to exert himself at peak capacity for several days. Superhuman agility, balance, bodily coordination, reflexes and speed, though not on the level of say Quicksilver. Deadpool is also highly unpredictable, making it very difficult for anyone to predict what he may do at any given moment.
Deathstroke: Due to severe injuries, had to undergo surgery and was injected with an experimental serum, turning him into a meta-human super-solider. His strength level is at least 5 tons (10,000 lbs), more than twice that of Deadpool. Slade also has heightened speed, agility, stamina and reflexes. All pretty much on par with Deadpool. Deathstroke's greatest "power" is his brain, capable of using up to 90% of it at any given time, thus making him a tactical genius, and able to turn his opponents own abilities against them. He does have a healing factor, but not on par with Deadpool. Slade could not heal his missing eye and cannot regenerate limbs, though he is able to heal from virtually any other wound.
Because of that insane healing factor & inability to die, Wade takes this point.
Deadpool: Typically a teleportation device, a holographic image inducer and a "magical satchel" that holds all of his unlimited weaponry and ammo.
Weapons: Guns, Grenades, Sai, Knives, and of course his Katana Swords. Wade can use virtually any weapon known to man and learn how to use it in minutes.
Deathstroke: Wears armor made of Nth Metal, a special metal found in the DC universe. Allows Slade to absorb blows from incredibly powerful beings, negate gravity, speed up his healing, increase his strength and it protects him against the elements and extreme temperatures. Weapons: Guns, Rifles, Knives and of course his Swords. He usually carries two, a giant Broadsword and a Katana. Slade's main Broadsword is made of "Volatile Promethium" A nigh-invulnerable metal, capable of absorbing near limitless amounts of energy. Slade also carries a staff made of Volatile Promethium, which can fire lethal or non-lethal blasts of energy, and can also bend or stretch to be used in his martial arts.
Due to the Nth Metal Armor and Promethium weapons, Slade gets the point.
Deadpool: Anyone that knows Wade Wilson, knows that he's bat-shit crazy. His healing factor actually makes him highly unstable, and prone to violent outbursts. Because Wade's healing factor was designed specifically to help cure his cancer, technically if his cancer was cured then he would actually lose his healing factor. His healing factor is also mentally driven, unlike Wolverine's "natural" healing.
Deathstroke: Missing his right eye could cause some depth perception issues but doesn't seem to affect him too much. Has a daughter that could possibly be used as leverage against him. Though incredibly powerful, his Nth Armor is not invincible. When his healing factor kicks in, it can render him temporarily insane and animalistic for a short time.
Due to Slade's healing factor causing temporary insanity, Wade gets the point.