I also think "Who was the first president." is correct, because it's past.
In my opinion is could be used here:
You have some cards/pictures of presidents (as a quiz or whatever).
Then someone asks you:
"Who is the first president?"
But that's pretty conjectured...
Another thing could be when you analyze history.
You check several years.
Then you could say: "Year x, Mr. x is the first president of America - something like a report.
(Sorry, I'm not very good in history )