Halep's achievements on all surfaces give her a very strong argument over Azarenka and Kvitova IMO, but there's a legit argument for Vika. I'd favour Halep to go on and achieve more in her career to remove any argument.
As a Vika fan, I'm not thrilled to say this but Halep is pretty clearly the best of the three (at least in singles). She has slams on two surfaces (and two tournaments), more finals than either, and leads the head-to-head with both. The other two have somewhat better non-slam resumes but Simona is no slouch there despite her recent run of crappy finals. And I wanted to say that Halep has had more shittastic performances but honestly the other two have had their share too :tape:. Vika dominated in her day but it was a fairly brief period and unfortunately she doesn't have a ton to show for it.
The OP should add Vika's and Petra's bronze medals though.