Overhyped
Power-Up Punch - I don't get it. It works on Mom, and to a lesser extent Tech Breloom, but I've seen people use it over Swords Dance and I just can't fathom why. The passive damage is mostly negligible, and you're better off SDing + attacking rather than PuP then attacking. Not a good move. It only works on a select number of Pokemon.
Assault Vest - This is a bulky offense item, not a defensive one. It allows you to add to a strength (Goodra, Ttar) or patch up a weakness (Conkeldurr). It doesn't work or is too gimmicky for much else. AV Blissey. Yes, I've actually seen this. What the hell, man. People just seem to want to slap this on anything without knowing the purpose of doing so.
Greninja - Solid OU 'mon and this is a case of shiny new toy syndrome more than anything else. Shouldn't be top 3 in usage, though this will probably settle down.
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Under hyped
Mega Blastoise - it's too bad he's a second tier mega otherwise people would be all over it. Spinner who can decimate spin blockers; sign me up. I get why he's not seeing much use, but this 'mon is really, really good.
Mega Pinsir - doesn't pull off sweeps with the ease of Luke or Megazard and also has crap defensive typing, but with good speed, excellent priority, good ability to take neutral hits, and holy crap 198 base power attack with zero draw backs, Pinsir shouldn't get lost in the shuffle. It's among the best sweepers in the hypothetical OU.
Noivern - not a great Pokemon, per se, but I don't think it should slip down to UU. Specs set is mostly outclassed IMO, though Noivern's move pool and good enough SAtk still allows it to be an offensive threat otherwise, and there's still that super fast Switcheroo and access to moves like Taunt and U-Turn.