Well, I voted no-one!!!
I think that Harry will be too involved with other stuff to have time to actually devote to a g/f... He's always busy with something or other, barely leaving im time to study, little lone date someone... notice, he only ever realyl though of Cho when he saw her, he just had too much other stuff on his mind, and I reckon that will only get worse!
Also, if he does date someone new, unless it's someone who is a part of their adventures, like Luna or Ginny, I can see them being very jealous of Hermoine lke Cho was, even though nothing is happening there (and I hope that it never does!!!)
Otherwise, possibly Luna, if she gets a little less strange, but I wrote about that somewhere else, so I wont go over it again!!!