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In The Lord of the Rings there are five Nazgûl that remain behind and attack the Hobbits on Weathertop.
Which Nagûl stabbed Frodo? Was it the Witch-king or was he one of the four that pursued Gandalf?
Crown as evidence:
While the answers are good and point to it being the Witch-king, the question Who were the Nazgul before they became wraiths? refers to kings plural therefore wearing a crown is only a pointer and not definitive. I'll wait to see if more evidence becomes available.
There were five tall figures: two standing on the lip of the dell, three advancing. In their white faces burned keen and merciless eyes; under their mantles were long grey robes; upon their grey hairs were helms of silver;... Two of the figures halted. The third was taller than the others: his hair was long and gleaming and on his helm was a crown. In one hand he held a long sword, and in the other a knife; both the knife and the hand that held it glowed with a pale light. He sprang forward and bore down on Frodo.
At that moment Frodo threw himself forward on the ground, and he heard himself crying aloud: O Elbereth! Gilthoniel! At the same time he struck at the feet of his enemy. A shrill cry rang out in the night; and he felt a pain like a dart of poisoned ice pierce his left shoulder.
Tolkien, J.R.R.. The Fellowship of the Ring: Being the First Part of The Lord of the Rings (pp. 195-196). Houghton Mifflin Harcourt. Kindle Edition.
but all blades perish that pierce that dreadful King. -Aragorn to Frodo about the incident.
The other four all had silver helms, only one of the five had a crown on top.
For even the Wise might fear to withstand the Nine, when they are gathered together under their fell chieftain. A great king and sorcerer he was of old, and now he wields a deadly fear. -Gandalf on the nine.
If you're ok with taking the movies' word for it, Gandalf clearly states that it was the Witch-king
Gandalf: Sauron has yet to reveal his deadliest servant. The one who would lead Mordor's armies in war. The one they say no living man can kill: the Witch-King of Angmar. You've met him before. He stabbed Frodo on Weathertop.
Two of the figures halted. The third was taller than the others: his hair was long and gleaming and on his helm was a crown. In one hand he held a long sword, and in the other a knife; both the knife and the hand that held it glowed with a pale light. He sprang forward and bore down on Frodo.
At that moment Frodo threw himself forward on the ground, and he heard himself crying aloud: O Elbereth! Gilthoniel! At the same time he struck at the feet of his enemy. A shrill cry rang out in the night; and he felt a pain like a dart of poisoned ice pierce his left shoulder.
Theoretically it could have been from another of the Nazgûl (we don't see the actual strike) but the implication is very clear.
In the Peter Jackson film adaptation, the event is rather clearer
IN THE TWILIGHT WORLD:
Frodo finds himself in the weird twilight world...he looks upon the Ringwraiths, now visible in their TRUE APPEARANCE: Five Ghouls dressed in long Grey robes, with white hair, and Pallid, ruthless faces. THE WITCH KING extends a haggard hand towards Frodo, reaching for the ring on his finger. Frodo's trembling hand extends forward as if by the pull of the ring...he slides to the ground, unable to pull his hand away. The witch king snarls and springs forward. He stabs at Frodo with a wicked dagger! Frodo winces as the tip of the dagger sinks into his shoulder. Suddenly, Strider charges at the RINGWRAITHS, wielding his sword in one hand, a flaming torch in the other. He moves in slow motion, visible through a sea of mist. Frodo sinks to the ground. Behind him is a faint image of a Ringwraith fleeing, his head engulfed in flames. With draining strength, Frodo manages to pull the ring off his finger...
And again in the Ralph Bakshi adaptation of Lord of the Rings, he's quite definitely stabbed by a character later identified as the Witch-King.
It was the lord of the Nazgûl (previously known as the Witch King of Angmar), or at least it seems to have been the most powerful black rider present.
Two of the figures halted. The third was taller than the others: his hair was long and gleaming and on his helm was a crown. In one hand he held a long sword, and in the other a knife; both the knife and the hand that held it glowed with a pale light. He sprang forward and bore down on Frodo.
Tolkien seemed to have a thing about height as a indicator of power. (Eomer was the tallest of the riders who accosted Aragorn, Legolas, and Gimli.) The way the figure with the blade is singled out strongly suggests it was Nazgûl lord.
