Who is the smartest?

I choose the Beast

Banner. Naturally.

He outsmarted Reed, Doom and Stark. He humiliated Start AND Beast.

Kang is smarter than Doom and Thanos is smarter than everyone and anyone

I'm going with Beast. He can rely on Shi`ar technology.
If he wanted to he could build a door that doesn't open and lock away any information.

Doctor Doom

Not everything is comic book movies here, it's non vs battle comic stuff too.

It says Comic Book movies.

Anyway, if this was D.C. it'd be a toss up between The Batman and The Flash.

I'd vote for Flash.

Tony Stark discovering how to make a new element and create new tech, then miniaturize it (arc reactor), then continually stay a step ahead of anyone copying his tech, etc.