I wouldn't say she was a more realistic villain than Voldemort, perse, but she was definitely more annoying. I wanted one to snuff it just as much as the other.
My Sophomore English teacher was extraordinarily like Umbridge, looks and all. It took every bit of restraint I had to not beat her with my textbook, and my fellow Potterheads in the class and I called her Dolores when we would talk about her.
I loved, loved Umbridge as a villain. She's a big part of the reason Order of the Phoenix is my favorite book of the series. And she couldn't have been more perfectly cast in the films.
I always hated Snape. I was supposed to. But I guess I never really saw him as a villain.
OotP is my favorite one too ^^ and i agree that she was cast perfectly. she was basically perfect for the role. her voice especially is perfect, she has the sickly sweet type of voice as in the book. i imagined her voice pretty darn similar to how i actually heard it in my head while reading.
i think i'm one of the few people who believed Snape was good... it totally killed me when he killed Dumbledore, i was like "nooooooo he TRUSTED you DDDDD;" and when i found out all about him and Lily i was like "wtf you ARE a good guy o_O" and then i cried because he was dead.
Voldemort is deffinetly a better villain with more and more of his evil showing. He's my favorite villain really. But Umbridge is deffinatly more evil and she should have died at the end or at least gone to Azkaban.
The reason why I thought Umbridge made such a awesome villain was because she's the kind of person you'd realistically meet in real life: sugary and perky but underneath the facade is a twisted, horrid person that shows more often than you'd think.