Boston Rob might have been my favorite if he had gotten more airtime. Like Jadewarlock said, he might be forgotten by viewers, too.
Parvati wasn't my favorite on her original season, mainly because she was like a caricature of herself. She's matured since then and, like Sandra, has a strong ability to read people. Unlike Russell (or Rupert, I guess), she doesn't let others' feelings for her determine her gameplay. Remember back to the merge when she said at TC that she felt like a leper since no one was talking to her! And, even now that she doesn't trust Russell, given his dumb move to vote out Danielle, she still doesn't have to get any sort of immediate revenge on Russell. I loved what Parvati said to Jerri during the reward; she said that Jerri is now "under her protection."
You know, I think this might be Parvati's strength: she is good at giving and granting (and receiving) loyalty. (Notice how proud Danielle was that she was "closer than you think" to Parvati.)
And, she does this in multiple ways. I think that's why some viewers found her flirtiness off-putting. Flirtiness might just be one method of showing that you want to be on someone's side.