Out of that list, Lady Gaga. By a wide margin.
Rihanna, Beyonce and Britney in their interviews...
I don't think Katy is dumb. Actually, I've seen her be quite witty and controlled in some interviews. It's a pity her career is so basic though.
But overall, Madonna. She's a genius - literally. She has an IQ of genius level, and she's a member of that group (gosh, I just can't remember the name
***) that they can be part of.
I think it's quite clear to see she's more intelligent than any of the others. She's very eloquent (giving you bitches new vocab every year) and has a great general knowledge and of the arts too.
Plus, she's been running her **** for over 30 years.
*******, I've got to Google that.