Mercy is still the best Medic. I checked, the only thing that was nerfed, was Mercy being able to get Double Rez after using her Ult. You know, when Mercy players Rez one person, then use their Ult which reset the Rez cooldown, then Rez someone else. That's happen now, and that's great.
But Mercy is still the medic you'd want, mainly for her Ult. Ana's ok, but let's not forget that she can only boost ONE person. Why would I want to boost just one person, when I can boost several, and keep the team on the point?