Overrated:
Hydreigon with Draco Meteor. I've met several of these guys in random wi-fi battles, and nearly all of them started out with using Draco Meteor, and none of them held the White Herb (I think I've only met 1 that didn't do so). They're dragons, and they have great special attack, but they can't OHKO everything with Draco Meteor.
Underrated:
Archeops. I'd really like to experiment with a double or triple-battle team showcasing Archeops and a good defensive skill-swapper. Should be interesting.
I think Dragon types are overrated. Yeah they're resilient, but Dragon is only super effective on Dragon. Kinda the same with Ghost but it's also super on Psychic.
Underated: At the moment, Raikou. I made a team around a 4 special attack Raikou, it's doing ok right now. Needs more testing but eh :/ There aren't too many OU Electric types, except Rotom-W, Jolteon, and Magnezone.
Overrated: Lucario.
Underrated: Cofagrigus.
Thats why it is part steel type
NOOOO SCEPTILE WHY IS YOUR SIGNATURE MOVE STOLEN FROM YOU NOOOOO
Rapidash is underrated, yet it's one of my favorite Pokémon. Its fast and it barely has a move pool good in enough to survive
Except for Altaria, the forgotten defensive dragon type.
The only other underrated Pokémon that I can think of is Fearow, and is an overrated Pokémon, I can't think of any that hasn't posted.
Overrated: People've already mentioned Dragon types, and I know I'm gonna get a bit of hate for this, but I think Salamence is overrated. It's got good stats, but it's so....generic. No special moves, nothing to make it stick out besides being a dragon. Also, its wings look like Pringles.
Underrated:
Liepard. With a high speed, thunder wave, and hell, sand attack, night slash, and fake out...it can work.
Bump of almost exactly 1 year.. got to shut it.