I'm with Casual Shinji on this. The term, to me at least, always seem to suggest that everyone is wrong for praising the things they like, and I don't like that type of thinking. There are stuff you like that most people don't, and then there's vise versa. I prefer thinking like that rather than using terms like "Overrated" and "Underrated"
In which case...
Elfen Lied- Considered a classic by most, bu I don't think it's very good. I don't think the idea is well executed, and outside of two characters, I found the rest either bland, useless, underused, or annoying. Then there's also the ending, which aggravates me. I've read the manga, too, which is a bit better than the anime, but still has its share of problems, as well as ann ending that kinda pissed me off. However, a lot of people are emotionally touched by it and do see the deeper meanings, and that's fine. I used to love it in high school, so I can see why others would.
Mawaru Penguindrum- This one isn't as widely seen, but most people I see who watched it end up loving it. I absolutely hated it, and I feel like I'm in an incredibly small minority. I couldn't stand the style or the characters, every line of dialogue was the same pseudo-philosophical drivel that you see in most every anime, coming out of every character (even kids), there was a flashback every few minutes, and it was clear that it was more concerned with symbolism and philosophical messages than it was telling a story, as thinking about the anime story-wise it makes no sense. I thought it was pretentious garbage, and this is coming from an Evangelion fan, and I'm not kidding when I say it's one of the worst anime I've sat through. It's well-made, and original, but it pissed me off more than most other bad anime.