Doctor Fate was first introduced in More Fun Comics #55. A young man who was helping his father on an exhibition, Kent Nelson discovered an ancient helmet that contained Nabu, a powerful sorcerer who imbued the former with gifts when he put the Helmet of Fate on.
As a main member of the Justice Society, Dr. Fate was the most well-known magic-user in the DC Universe. Until 1985, at least. John Constantine was originally a side character in Alan Moore's Swamp Thing run before he earned his own Vertigo series, Hellblazer. Constantine's foul mouth and cynical, snarky personality allowed him to quickly become a very popular character. But who is more powerful?
10 Fate: Lord of Order
It's worth noting that Doctor Fate's title as a Lord of Order already positions him as one of the most powerful mystical beings in the universe. The Lords of Order use their powers to fight against the Lords of Chaos in a never-ending mystical battle.
The battle was originally a simple fight between good and evil but has evolved into a necessary quarrel to balance stagnation and anarchy. The Lords of Chaos' main goal is to bring onto the universe "kali yoga," an ancient term that describes a period where chaos rules all.
9 Constantine: One of the most powerful Occult Magicians
While Doctor Fate draws his power from hosting a powerful sorcerer, Constantine is an expert in occult practices. The main difference is that Constantine is a normal human being, who has become an expert in magic through holistic study.
As an occult detective, Constantine relies on his wit and cunningness to achieve his goals. Unlike most magicians, he rarely uses magic and only casts spells when in situations that desperately call for them.
8 Fate: Consistently beats an extremely powerful archenemy
As a Lord of Order, Fate's main enemies are the Lords of Chaos, and there's no Lord of Chaos that has been as much of an issue to Fate as Mordru. Mordru is a powerful entity that is said to have existed before the beginning of the universe and will survive its end.
During the JSA arc Princes of Darkness, Mordru switched bodies with Doctor Fate and was able to steal the Starheart from the Green Lantern Alan Scott. Mordru then joined the demon Eclipso and Scott's son Obsidian in covering the entire world in darkness. Fortunately, the JSA defeated him, and Mordru was imprisoned in the Rock of Eternity.
7 Constantine: Consistently outwits powerful angels & demons
Forever the neutral anti-hero, Constantine is a pain in the ass to both the inhabitants of Heaven and demons from hell. When Constantine was diagnosed with lung cancer, he came to the angel Gabriel for help. When he refused, Constantine hatched a plan that resulted in Gabriel becoming a fallen angel.
Turning to Hell with his predicament, John sells his soul to two different lords of Hell, the Second and Third of the Fallen, neither knowing that the other one has bought his soul as well. Knowing that they can't share the soul, Constantine's death would trigger a civil war in Hell, so his cancer is cured and he gets off scot-free, albeit not without flipping his victims the bird.
6 Fate: Far more powerful arsenal
Most of Fate's abilities come from the special items of power he has in his possession. As mentioned before, the Helmet of Fate was created by Nabu and holds his spirit. Nabu also granted Kent the Cloak of Destiny, a fireproof, magical cape that Doctor Fate wears.
Fate also has the Amulet of Anubis, which is the circular item seen on Fate's chest and shoulders. It allows the user dimensional travel and holds a small fraction of the death god Anubis's own power. When Kent died, he and his wife Inza's souls were trapped in the Amulet.
5 Constantine: Far More Intelligent
Constantine's true power comes from his intellect. He's a master tactician, able to come up with strategies under pressure and off the cuff that result in him getting what he wants. He's been described as a master manipulator and is skilled in the art of "sleight of the tongue"
When Constantine faces a powerful opponent, he doesn't use countless spells to try and defeat them, but he tricks them into putting themselves at his level. During the Dangerous Habits arc, Constantine tricks Satan into drinking Holy Water, essentially poisoning him, and begins beating the crap out of him with a glass bottle.
4 Fate: Has gone toe to toe with the Spectre
Any fan of the DCU knows the Spirit of Vengeance very well. The Spectre is a godlike antihero who possesses human beings in his quest against the wicked. He's the Wrath of God personified, and while his original host was Jim Corrigan, he's taken many human hosts, such as Hal Jordan.
Doctor Fate fighting the Spectre might seem like a suicide mission, but he actually held his own against the spirit. Nabu took control of Fate's body and faced off against him. He knew he would die, but his goal was to unnerve the Spectre enough for a different host to hold him, which he succeeded in.
3 Constantine-Powerful Body Modifications
Constantine is almost a completely normal human, but if he can think of a way to give himself a magical advantage, he'll take it. To protect himself against the Lukhavim, demons created from dead souls, Constantine got over his fear of needles and got a multitude of tattoos across his entire body consisting of magical symbols and words of protection.
Constantine's blood is also unique, tainted due to a blood transfusion caused by one of his enemies, the demon Nergal. Having relations with a succubus tainted his blood even more, but gave it magical uses. Constantine's blood has healing properties and was used as a deterrent against the King of Vampires.
2 Fate: Host of Nabu
Nabu is the Lord of Order that inhabits the Helmet of Fate and created the items Doctor Fate uses to fight crime and maintain order. Nabu himself is a powerful sorcerer, born over 10 billion years ago. After living in the mystical world of Cillia, he traveled to Earth and manifested himself a human body, becoming Nabu the Wise, and advisor to Ancient pharaohs.
When he was the adviser to Ramses, Nabu faced off against the Spectre for the first time, who killed Ramses for his crimes against the Hebrews. After creating many powerful objects such as the Claw of Horus and the Scarab of Kha-Ef-Re (the Blue Beetle scarab), Nabu put himself into a trance and hibernated in a tomb, waiting for the Helmet of Fate to be found.
1 Constantine: Killed Nabu
While Nabu was killed by the Spectre force in the pre-Crisis continuity, the New 52 Future's End event showed a possible future where he met the same fate at the hands of John Constantine. After stealing the Helmet of Fate, the Helmet threatened John in many ways, but as it was hostless, it wasn't much of a threat.
Constantine challenged the Helm in the Auditorium of Anubis, where the god watches over the two in judgment. After tricking Nabu into revealing that he never cared about anything but himself, Anubis picks Nabu up and eats him.