1. Mr. Fantastic
He is often stated as the smartest man on Earth, by pretty much everyone except people known to be arrogant like Doom and sometimes Tony Stark. He has galactic level intelligence and there's basically nothing he can't do with his mind.
2. Doctor Doom
He is the only one who may even be smarter than Reed, but his ego and pride have always held him back. Most notably, he is the only person who can stake a claim to being both the best scientist/inventor as well as greatest sorcerer on Earth.
3. Hank Pym
The "Scientist Supreme", one of the most intelligence Marvel characters. He is known for his low self esteem and occasional mental instability, but he is also a true super-genius who acn be unfairly downplayed.
Although Bruce Banner is often under estimated, due to being more famous for his brawn, he has, in recent years, been able to show off his intellect more. He may not be number 1, but he is certainly one of the smartest geniuses alive, as shown by his plethora of Bannertech inventions such as force fields.
For a long time, Peter Parker had to make do with the limited resources he had access to, creating potential world-changing inventions on a shoestring budget. However, in more recent times he has been able to take advantage of the opportunities which came his way, and we've been able to get a glimpse of his potential as he created amazing technology ranging from high tech armors to reverse-engineering weapons created to combat Celestials.
6. Black Panther
Not only does T'Challa rule the most technologically advanced country on Earth, but he is also probably the smartest person in Wakanda. Backed by a team of scientists, he can create pretty much anything he puts his mind to, and was even the creator of his own field of science - Shadow Physics.
7. Iron Man
Although Tony tends to focus more on technology and upgrading his armor over all else, he is certainly a true expert in all other fields of STEM, ranging from biology to theoretical physics.
8. Blue Marvel
Although Adam Brashear isn't quite as well known as most of the people on this list, he has built up an impressive resume as a scientist in his few appearances - most notably, playing an integral part in transforming Galactus himself from a Devourer of Worlds into a Lifebringer.
The X-Men's resident genius, Hank McCoy is one of the smartest men on Earth, for sure. His feral appearance hides one of the brightest minds on the planet, especially in fields like biology and technology in general. Some of his inventions include amazing devices like time machines, which is why he earns his place on this list.
10. Amadeus Cho
Most well known for his brand of "hyper math" - calculating trajectories and doing math in his head to use as a combat tool, Amadeus Cho is one of the smartest people on Earth. Although he has been unduly hyped by spouting he is 8th smartest in the world all the time, he does get a spot on the list, if not where he says he is.