This list only consists of people who are dangerous because of their prep ability, and born genius-intellect....therefore, no mutant(Forge, Prodigy), no chemical enhanced metahuman(Deathstroke, Leader)
8) Tony Stark - Probably the 3rd Smartest man on Earth, Super-genius, but alcoholics costed him a lot of trouble. He is smarter than most people on this list...but just not as dangerous as they are.
7) Black Panther - stealing the cosmic power, crazy advanced tech. Slight edge over Stark.
6) Ozymandias - defeated a god-like being who can foresee the future.
5) Batman - World recognized "King of Prep" , defeated countless of powerful opponent, He maybe not as smart as Stark in pure scientific knowledge, but his dedication and his obsession over perfection making him the most The Most Dangerous Man on DCEarth. "Always few steps ahead"! BATMAN!
4) Reed Richards - Arguably the Smartest Man on Earth, taken down God-like being, "Mad-Scientist" , He is definitely deserves this place on this list.
3) Lex Luthor - He is one of the best scientist in DC, built all-kind of stuff to allow him to go toe-toe with Superman, Became the President once, even though he is widely recognized as villain. Almost become a God. He is truly the Greatest Criminal Mastermind in DC.
2) Dr. Doom - He has the scientific knowledge of Reed, the inventing ability of Stark, the Resources of Black Panther, the Strategical mind of Batman....and also master in Mystical Magics, making him one of the most powerful sorcerer on Earth, only second to Strange. Stole from Omnipotent. Trust me, He can conquer the Earth whenever he likes.
1) Thanos - the reason is obvious. He is on this list not because of his superhuman power, in fact, his mind is what makes him so dangerous. He preps for preps, he would makes Batman feels like a little boy scout when it comes to Prep.
This is all based on my own Opinion, I hope you all too can make your own list. ALL CRITICISMS ARE ALLOWED.