The Witch-king and Gandalf are both powerful, but Gandalf is a Maiar, making him more potent than most beings. In the extended edition for Return of the King, Gandalf struggles against the Witch-king, serving as added tension before Eowyn's victory.
The Witch King was certain that he could defeat Gandalf when they faced each other at the shattered Gates to Minas Tirith but he was basing this on the strength Gandalf had shown earlier at Weathertop. Even with the boost in his powers it is likely that Gandalf could have beaten him there.
Gandalf and the Whitch King ( Head Nazhul) stood of facing each other, but the Witch King fled when the Rohirram arrived at the Pelennenor Fields. Y-e-s … Gandalf was able to defeat the Witch-King in all their encounters on record.
In the rare manuscript The Hunt for the Ring: Time Scheme - Black Riders, Tolkien speculates that the Witch-King might be afraid of Frodo, as he invoked Elbereth (a “name of terror for the Nazgûl”) and he could have slain the Barrow-wight.
Summary. Gandalf, as an Istari, was sent to Middle-earth to assist in the fight against Sauron, but he was forbidden from using his full power to interfere with the natural order. Saruman's downfall serves as a warning of the dangers of exceeding the limits placed on the Istari by the Valar.
As a wizard, Gandalf's magical power is in light and fire, which he is able to manipulate by casting beams of light, starting and directing fires, and more. Although he is very powerful magically, his power does have its limits.
As with others, the desire to understand the workings of the One Ring can lead to corruption, much as a glance at the One Ring lead Smeagol to murder his friend. Saruman in many ways was far more vulnerable to this than the other Istari, and his growing pride also made him weaker.
'It was a Balrog of Morgoth,' said Legolas; 'of all elf-banes the most deadly, save the One who sits in the Dark Tower. ' A Balrog is definitely stronger than the witch king.
Secondly the Witch King was given an added demonic force by Sauron against Gandalf the White. Sauron was a far more powerful angel than Gandalf the White (yes Gandalf was an angel). And it is possible that even a small part of Sauron's power was enough for Gandalf.
Despite all his strength, Smaug does have one weak spot: an open gap in his treasure armor upon his left breast. One direct hit with an arrow or sword in that specific spot, and the dragon goes down. Even without magic, as long as Gandalf has the upper hand and Smaug is distracted, he can kill the dragon in one shot.
As previously stated, Gandalf is a Maia and is therefore superior to Elves regarding power. Still, this isn't to say that an elf couldn't develop enough skill and power to defeat a Maia. As a bearer of one of the Three Elven Rings (Nenya), Galadriel would have an edge against Maiar like Saruman and Radagast.
Saruman was a higher-ranked wizard and the leader of the White Council, while Gandalf was primarily seen as a wise warrior and counselor to Frodo. Gandalf became stronger as the White Wizard and the leader of the wizards, ultimately besting Saruman in combat due to his loyalty and growth in power and authority.
Galadriel's powers were so vast that it's said that other powerful beings like Saruman or even Elrond could not fully comprehend them. The powers she did have were used for good, of course, such as protecting Lothlórien and even toppling Dol Guldur, a fortress that was enhanced by Sauron's magic.
Bloodlines are very important in Tolkien's stories, and it makes a lot of sense that Sauron would fear the great-grandson of the man who cut off his hand. Aragorn carried the blood of Numenor with him. His genetic connection to the heroes of the Second Age established him as a dangerous opponent of Sauron.
Sauron was the great evil, Saruman is petty evil. What's more humiliating for Saruman, is that he was now beneath the notice of Gandalf. Gandalf parted with his Hobbit friends before they entered The Shire, instead going to the Old Forest to treat with Tom Bombadil.
Gandalf is no mere man, but a Maiar, which are essentially the equivalent of angels in Tolkien's world. Not only that, Gandalf is capable of feats of sorcery that could easily beat back the Dark Lord. Fighting the Balrog for three days and two nights makes everything else seem like a cakewalk.
In close-quarters combat, Gandalf easily has an upper hand because of the weapon he wields. Glamdring, the Elven blade he uses, would put Dumbledore in a spot of trouble. He'd need to maintain his distance at almost all times during this battle, while Gandalf would be comfortable fighting at both short and long ranges.
Gandalf by a long shot. Harry Potter is, at best, an above average wizard, whose best at disarming his enemies. Gandalf was a Maia, an angelic being. Harry Potter is just outclassed by a wide margin.
Galadriel has very strong magical powers, and she is said to be the greatest of the Ñoldor after Fëanor. The majority of her powers come from her Ring of Power, Nenya, the Ring of Water.
Saruman had always been jealous of Gandalf, and knew that he was keeping secrets from him, such as his suspicion about Bilbo's ring and Narya, secret. Gandalf also suspected Saruman of plotting to gain the One Ring for himself, and hinted at this at a meeting of the White Council.
Her ring was Nenya, the Ring of Water. Conscious of Sauron's power, and wishing to thwart it, she did not use the powers of her ring as long as the One Ring was in Sauron's hands.
