Most likely this guy:
In Harry Potter Universe, Snape can be considered something of a genius.
He is an extremely accomplished wizard academically.
Not only was he a gifted potion maker to the point of knowing better than the official schoolbook, but also was able to invent his own spells and use them with great efficiency.
Snape is possibly the strongest Death Eater. He is the only one among them who was able to learn how to fly without a broom, something only Voldemort was capable of in the books.
He was also a capable duelist. never once really being defeated. He is also a master Legilimens and Occlumens, feats that are mostly avaiable to those who can keep their emotions in check and have great self control.
Ultimately, his skills at deception were remarkable. He was able to play the double agent role to perfection, fooling Voldemort completely but also fooling the Order of Phoenix. Secretly aiding Harry and pretending to be a dutiful Death Eater wasn’t easy but he did it.
Snape was a magnificient wizard. Dumbledore was likely more brilliant than he was, and Voldemort was definetly more powerful but with Snape we have that perfect intellect being put to use in almost every field in his life. The feat of fooling Voldemort alone, for so long is remarkable.
Nevertheless, I acknowledge that Dumbledore is probably smarter than him but I will stick with Snape nonetheless.
EDIT: I honestly have never seen such a biased fanbase. I have written on topics of comics, films , Star Wars etc, but honestly this is the most biased group of people I’ve seen online. I haven’t even said that Snape is the most powerful wizard in the Potterverse neither that he’s the greatest. But immediately, people would degrade even what accomplishments he did have that are simply there to demonstrate his keen intellect. The dude was smarter than most in Potterverse, that much is clear. But sure, that’s plot armor for haters. Every fictional character has “plot armor” so get over it. And if you don’t keep it respectful,you will be deleted.
Cheers.