These are my picks but I have no real ranking for them since they're all geniuses in their own right.
1. Dr. Doom, Tony Starks, Reed Richards, and Bruce Banner. They formed a quartet of geniuses (called the Knights I think) during the Heroes Reborn saga. They all regarded each other as peers in terms of intellect. This was even acknowledged when they all returned from the HR universe and reunited briefly.
2. Tyrannus: When he came back in the Abomination's body he was able to work out problems that banner acknowledged he was having great difficulty with. Definitely underated outside and even inside the Hulkverse.
3.The Leader: He invented a machine so one could travel at the speed of thought. he built a time machine I think around the time Banner gained control over the Hulk. He built a device able to revive dead body cells.
4. High Evolutionary: Master of evoultion. I don't know all of his scientif feats but assume he deserves a spot.
5. Apocalypse: If not being a racist since he's a mutant and consider him human then he makes the cut. I know from his run in with Hulk he knows a lot about Celestial technology so he should be pretty smart.
6. Dr. Strange: If being racist and excluding Apocalypse but not being totally bias by acknowledging mystical smarts as being as equal to computer smarts then Stephen makes the cut. He truly is a great intellect being able to defat beings much greater than mere mortals can ever comprehend.
Other notable mentions are Doc Samson since he was able to separate Banner from the Hulk. I think Xavier os an expert in genetics but not sure. Pym discovered Pym particles and I think built Ultron. Tchalla supposed to be pretty sharp too. I just consider them below the top tier elite.
I consider Cho to be an upstart intellect wise like how Sentry is power wise. Cho has not done anything I know of that would warrant him being among the Eight.
Cho figured out how to kill the supreme god of the Skrulls during Secret Invasion. Thats enough for me. Been to hell and back. that seconds it