Throwing in my two cents.
Overhyped:
Sticky web. At first, it sounded like some kind of game changer that would do more than stealth rock. Revenge killing would be dead. Every team would have to run a sticky webber or rapid spinner to even the odds. Then it turned out only two viable Pokemon got it (Smeargle and Galvantula), people were prepared for it, and there was the defog nerf. This is more of a gimmick now.
Goodra. The resident psuedo-legendary of this generation was hailed as a god, but all it has is good special defense. It also lacks recovery outside of rest, and its offensive presence isn't that menacing.
Malamar. With the ability to set up contrary superpowers better than spinda, stop set-up sweepers in their tracks, and be a hard counter to king's shield, Malamar seemed like a great choice...until its stats were revealed to be quite lackluster. I'd say Malamar is still a choice, but it's not top OU material.
Physical Aegislash. It has 150/150 defenses to set up swords dances, a terrifying 150 attack when it comes out of shield form, and it also has priority, perfect ghost/fighting coverage, and a better protect. Aegislash's physical set is sadly very predictable, and doesn't bring the bang for the buck that it seemed to promise.
Parting shot. It sounds like a better memento as it lets you keep your pokemon alive, gives you momentum, and cripple the opponent. Only two viable pokemon learn this move, and neither have astounding stats.
Underrated:
Diggersby. It was dismissed as another com-mon until it was revealed to have Huge power. With more attack than Azumarill, a STAB quick attack, swords dance, eq, and a ghost immunity, Diggersby can be quite the surprise fighter in many occasions.
Talonflame. Okay, it's not underrated now, but it was underrated before. It looks to be a worse Swellow with worse typing that dooms it to being in low NU. Come on, even if it does get gale wings, that 81 attack isn't doing much, and it has a 4x weakness to stealth rock. With the defog buff, this thing can get some usage, but it's still a gimmick right? Turns out a STAB 120 base power priority move hurts a lot, and Talonflame also has the speed to U-turn out of threats before they kill it. Now it's one of the top used pokemon in OU.
Yveltal. What? A uber here? Believe it or not, there was quite a bit of talk about how Yveltal might take the same path as Kyurem-Black, but look at it now, still in Ubers because of its great stats, coverage, and moves.
Mega Blastoise. Why aren't people using this more? Sure, it looks like a slightly buffed up Clawitzer without an item, but Mega Blastoise tanks hits great, and it hits back like a tank. Definitely an under-appreciated mega.
Mega Kangaskhan. Before this got banned or extensively used, people were suggesting fun gimmick sets for this like paraflinch or special Mega Kanga. It wasn't until this became used did how broken it was become apparent.
Mega Pinsir. Probably dismissed because of 4x weak to stealth rock, Mega Pinsir sits at a decent 105 speed tier, and hits really, really hard with aerilate boosted priority and a flying type outrage.