Constitutionally and in reality, who is most powerful in the US? A Congress man (woman) or a Senator?

The President.  The expansion of Presidential powers has been a long-running and troubling thing that people in BOTH parties should be dismayed by.  Whether because you were aghast at Obama's use of those powers or Trump's, we should all be able to agree that one person should not wield the kind of king-like powers we have seen the executive branch assume. 

But, if we're talking about congress only, the Speaker of the House is the most powerful single congressperson.  They completely control the agenda in the House.  The Senate majority leader is much more limited due to the existence of the fillibuster and other Senate rules.  The Speaker however is basically the Dictator of the House.

If we're talking about just a regular Representative vs a regular Senator in a non-leadership position, then the Senator is clearly more powerful because there are fewer of them and because Senate rules give rank and file Senators more power than in the House.

Senators, that's why their term is 6 years. they also make and craft law. House members just vote on it. Senators also head or chair the committees. 

I HOPE this is for your Civics class; otherwise, you're REALLY deep in it, nome sayin'.