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.
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.
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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.
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Thats why it is part steel type
NOOOO SCEPTILE WHY IS YOUR SIGNATURE MOVE STOLEN FROM YOU NOOOOO
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.
Liepard. With a high speed, thunder wave, and hell, sand attack, night slash, and fake out...it can work.