Nancy Reagan's megrim was drugs, especially child drug use.
Hillary Clinton's obsession was everything (well, hey, that's why she went into office for dolo: so she could wrap up the bits of everything she'd left unaddressed as Dame the First).
Laura Bush's are the vagaries of peril that confront American boys (WTF?). I guess there's also using her intelligence as distraction from her husband's nescience.
What was Barabara Bush's? I can't think of anything.