I chose Umbridge and Molly, not because I believe they would be a good couple (which I don't), but because in all honesty, Molly deserves that little toad, they are both terrible. The worst mother with the worst professor, fair match.
Now, for Ron and Harry... I used to ship them, but as I read the books I slowly started shipping Ron with Luna and Harry with Ginny, I still do ship them, but I mostly involve the Ronarry ship in fanfics where they are acting like parental figures to Hermione and she gets annoyed with them because of it.
Nagini and Fawkes... No... That's all I can say is no.
Voldemort and Snape?! That's an even bigger no! If they were in their teenage generation, I can see it sort of making sense, but as adults? Hell no! Plus, that would only give Bellatrix another reason to hurt Snape.