Not all villains are created equal. There are 23 main villains in the Marvel Cinematic Universe. There are actually more than that if you include secondary villains. Some of them are geniuses who spend their entire lives working on their evil plan. Others aren't quite so organized.
Some villains want to take over the world. Others want to destroy it. There are even a few with more personal vendettas. Whether they succeed in these endeavors or are foiled by the Avengers, even the worst plans and planners deserve a spot on this list.
10 Smart: Killmonger
N'Jadaka Erik "Killmonger" Stevens (Michael B. Jordan) spent his whole life developing his plan of revenge on Wakanda. His father was murdered by T'Challa's father when Killmonger was young, but instead of being taken in by his royal relatives, he was left to fend for himself.
Killmonger seeks revenge for his father's death. He also plans to distribute Wakanda's advanced weapons to the disenfranchised people of color around the world and start a revolution. While some of his ideas are noble, his plan is a threat to the status quo and to lives of thousands, making him a villain. As a villain, he's got an organized plan with an intellectual backing.
9 Dumb: Alexander Pierce
Alexander Pierce (Robert Redford) was the Secretary of Defense when he met Nicholas J. Fury. They were involved in a hostage situation in Bogotá. After Fury saved Pierce's daughter, the two became colleagues. Pierce secretly joined Hydra and facilitated their takeover of the United States government and S.H.I.E.L.D.
What puts Pierce on the dumb list is that he then hired Fury as Director of S.H.I.E.L.D. Fury then ran the operation that took down Hydra. Essentially, Pierce facilitated his own defeat by hiring Fury. Not only that but Pierce sent James Buchanan "Bucky" Barnes as the Winter Soldier to take down Captain America, his childhood best friend. Pierce may be a successful politician, but he's not a very smart villain.
8 Smart: Loki
Loki (Tom Hiddleston) is a big baddie in more than one Marvel movie. He made his debut in Thor (2011) as Thor's trickster brother. He is troublesome and conniving and incredibly charismatic. He fully became the villain in Avengers (2012) when he brought a race of aliens to invade Earth.
Loki isn't as focused as Killmonger. He has more of a chaotic energy about him. While his major plan failed, he succeeded in wreaking havoc on the helicarrier among the Avengers. He's smart, but the Avengers are often smarter.
7 Dumb: Ronan The Accuser
Ronan the Accuser is a Kree warlord. In Guardians of the Galaxy, he makes a deal with Thanos to obtain an infinity stone in exchange for the destruction of Xandar, a planet he despises. He betrays Thanos after seeing how powerful the stone is, and decides to keep it for himself.
Keeping the stone gets him defeated by the Guardians. Double-crossing Thanos is never a good idea either. Not only did Ronan's plan fail, but it was also barely even his plan in the first place. Unfortunately, Ronan just isn't the brightest villain in the bunch.
6 Smart: Mysterio
Quentin Beck "Mysterio" (Jake Gyllenhaal) is a former employee of Stark Industries. He is a technological genius with a team of other intelligent SI employees behind him. Beck didn't feel appreciated by Tony Stark and was ultimately fired. He vowed to have his revenge.
Once Stark died saving the world from Thanos, Beck had to get creative with his revenge plan. He chose to take out his anger on Stark's protegé, Spider-Man. He used his holographic tech to make himself a superhero. Though he was taken down by Peter Parker, he successfully released a message revealing Spider-Man's identity and framing him for the destruction from his ruse.
5 Dumb: Thaddeus "Thunderbolt" Ross
Thaddeus "Thunderbolt" Ross was the Army Lieutenant General in charge of recreating the Super Soldier Serum. That's the project that turned Bruce Banner into the Hulk. After Banner's escape, Ross turned his attention to Emil Blonsky. He ignored all safety protocols and turned Blonsky into the Abomination.
This all means that in The Incredible Hulk, Ross both created the hero and a villain, who ended up destroying half of Harlem in their fight. Ross isn't a classically evil villain, but he is a man whose dedication to rules and victory sometimes gets in the way of his morals. However, his desperate allegiance to the powers that be causes him to be a little less than intelligent.
4 Smart: Helmut Zemo
Helmut Zemo (Daniel Brühl) was a Sokovian intelligence officer. When the Avengers fought Ultron in Sokovia, Zemo's father, wife, and son were killed. Zemo blamed the Avengers for his losses. He carefully planned his revenge.
Zemo sought to destroy the Avengers from within and fracture their team. To do this, he framed Bucky Barnes as a terrorist and revealed that Barnes killed Tony Stark's parents when he was the Winter Soldier. Steve took Bucky's side against Tony, splitting the Avengers in two. Zemo is one of the few villains who succeeded in their plan.
3 Dumb: Ego
Ego (Kurt Russell) is a Celestial, a powerful being that existed before other life in the universe. Ego found himself disappointed by the life that developed. His plan was to plant seeds of himself on planets around the universe and turn them all into extensions of himself. He also wanted a companion, so he impregnated many different women to create a child. None of them before Peter Quill had the Celestial power, so he killed them.
Ego's dumbest moment was revealing to Peter that he killed Peter's mother by planting a tumor in her brain. Peter was on board with Ego's plan, mostly due to Ego's mind control. But hearing that admission caused Peter to break the mind control and turn against him.
2 Smart: Thanos
Thanos (Josh Brolin) is an alien who believed the universe needed to be cleansed of half its population to achieve balance. He moved from planet to planet, enacting his genocidal plan. He also searched for all the infinity stones so he could simply snap half the universe away.
Thanos defeated the Avengers and successfully achieved his goal. Even though his actions were ultimately reversed, his organization, dedication, and focus make him one of the smartest villains out there. Even the Thanos from the past was on the same page and took the original plan to the next level.
1 Dumb: Justin Hammer
Justin Hammer was a military contractor and CEO of his own technology company, Hammer Industries. HI was a rival of Stark Industries. Hammer attempted to sully Stark's reputation, but only ended up ruining his own. In a last-ditch attempt to elevate himself, he brought in Russian criminal and physicist Ivan Vanko to help him design his own arc reactor.
His plan backfired dramatically. Vanko managed to create a drone army without Hammer noticing. He then betrayed Hammer to enact his own evil plan. Hammer's reputation was permanently ruined and he was imprisoned for assisting Vanko. Hammer's ignorance makes him the dumbest villain in the MCU.