Peyton is not underrated at all. Nowadays she gets more love than Brooke IMO (with good reason, Peyton changed for the better, Brooke for the worse) also I think Haley is more underrated than Brooke and Peyton. I don't hate Peyton at all I just disagree with the fact she's always being labelled as an underrated character when at present she's getting more love than hate
IMO underrated characters are
Jenny Humphrey (GG)
Katie Fitch (Skins)
Jessica Angell (CSI NY)
Angela Montenegro Hodgins (Bones)
These girls barely get any love even though they are great characters. Not to go on but compared to Peyton these girls get no love
Different strokes for different folks and all that jazz, but
from my perspective I cannot not find these characters interesting/likable, whereas the majority of fandom disagrees:
The Tribe - Amber and Danni. The former largely hated, the latter almost completely hated. I think they are among the best things that show created. Nothing beats idealistic female ass-kickers. Extra points for being flawed, having weaknesses, making mistakes and altogether being human and relateable.
Charmed - Prue. Imo, a somewhat similar type of character as Amber and Danni I've mentioned above. My very favourite type of a character that most people seem to can't stand.
Lost - Ana Lucia. Another flawed and powerful lady. I was genuinely surprised to find so many fans hated her. I though she was awe-striking. Also Claire. But here I mean primarily by the writers. Somehow I got the idea they saw her as an irrational chick who's only role was to take care of her baby and have panic attacks.
Jericho - Emily. Again, I was surprised to learn about the massive hatred. I personally thought she was just another extremely likable Jericho character, as most of them are to me.
Dawson's Creek - Jen. Again, by the writers. They so clearly preferred Joey. Their right, but for me it was the other way round.
OTH - Peyton. Her character is one of the few things I actually find compelling about the atrocity that is OTH (atrocity imo:).
Gossip Girl - Vanessa. With the development of her character I also have a bit of an issue (especially lately), but the premises of her character are more interesting and relateable to me than those of other female characters on that show.