Matt & Rebekah are OK in my opinion, but I don't think they're underrated. Almost every fan of the show loves them.
Stefan & Damon are one of my favorite relationships on the show, but they're not underrated. They have many fans.
Stefan & Elena are not underrated. True, a bigger part of the fan base prefers DE over them, and they're probably the couple with the most haters, but they're still one of the most popular couples on the show, and have a big & loyal fan base (Just IMO, maybe I'm a little biased because I'm DE).
Tyler & Caroline were amazing in season 2 IMO, and that was when most of the fans liked them, so they weren't really underrated back then. They weren't great in seasons 3 & 4, so although they did loose a huge part of their fan base over those seasons, I can't really say they're underrated IMO right now.
Jeremy & Bonnie were OK in seasons 2 & 3, I didn't really care much about them (but I hated how Jeremy cheated on her with Anna). This season, however, they really grew on me, so I had to pick them.

And Forwood. I don't really think the others are underrated...