Because there was again a lot of discussion about individual duelling skill in other threads and it's sort of derailing, let's discuss here.
This is duelling skill, so a combination of speed, reaction feats, diversity, spell control, the ability to use non-verbal magic, etc. This is NOT spell creation, transfiguration, potion creation, apparition, broom flying, etc. Purely how these people stack up in a duel.
Everyone is as they were shown during the book series, not their prime, unless specified.
The below order shows how I rank these wizards and witches. Feel free to give your own or comment on my your ranking:
* Dumbledore (one of the greatest wizards ever, only one Voldemort ever feared, master of non-verbal magic, blitzed multiple opponents at once, took down the notorious Grindelwald)
* Voldemort(Almost defeated 3 highly skilled wizards at once, fought on par with Dumbledore while he had the elder wand)
* Grindelwald (prime, present time-line is featless) (fought a duel for hours with Dumbledore in his prime)
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- - - -
* Delphini Riddle (TCH) (Exact placement is somewhat difficult. Fodderized (near-)prime veteran auror Harry Potter, displayed enormous magic skill such as unassisted flying and wandless tk, briefly held off Harry + Ron + Draco + Hermione before going down)
* Bellatrix Lestrange(took down 4 wizards by herself with ease, defeated Black and Kingsley back to back and blocked a spell from Dumbledore, killed Tonks, was winning a 3 on 1)
* Kingsley Shacklebolt (took on and defeated two death eaters at once, reacted to Dumbledore's blitz, took down 3 death eaters while on broom, duelled against Voldemort)
* Antonin Dolohov(almost defeated the trio and Neville with non-verbal magic, was winning against Black, almost defeated the trio in book 7, defeated Moody, defeated and killed Lupin)
* Filius Flitwick (former duelling champion, charms professor, took down Dolohov)
* Severus Snape(fought on par with McGonagall for a little while, magical prodigy who invented different spells, legimency and occlumency master, stomped Harry in book 6)
* Minerva Mc Gonagall(stalemated with Snape, took on Voldemort together with 2 others, said to be extremely gifted from birth)
* Sirius Black(defeated an unnamed death eater, fought on par for a short while with both Dolohov and Lestrange, said to be extremely gifted)
* Horace Slughorn(held in high regard by Dumbledore, duelled against Voldemort)
* Remus Lupin(defeated Lucius Malfoy during the battle of the MoM and was one of just three good guys to be unharmed, reputation for being a very skilled wizard)
* Yaxley (stalemated with Flitwick, one of the last death eaters to go down during the battle of Hogwarts)
* Alastor Moody (prime should be way higher, but lost a lot of his shine) (reputation for being one of the best aurors ever, put countless death eaters in Azkaban, tons of experience)
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Mid tiers and below.
Too many to count. Harry, Draco, Hermione and Ron should probably be near the top, the mid tier is filled by a lot of death eaters (like Lucius Malfoy, Barty Crouch Jr and possibly Thorfinn Rowle), most aurors (Tonks, Dawlish, probably the Longbottoms) should be here, as well as some professors.
Almost all students and even adult wizards should be bottom mid tier or low tier, since they do not have any duelling experience. Fred or George said that most adults at the ministry of magic could barely conjure up a shield, which is entry level stuff.