Overhyped: Fairy-types. Mostly specially based on the offensive and defensive sides, but their low powered moves make them less than stellar coverage moves. Azumarrill has been the only Fairy to be effective so far but lets not forget about kl...woah, i'll get to that later.
Mega Mawile: everyone was running in fear of this thing since its power is ridiculous but it is way too slow to do much while its priority doesn't have enough PP and can be played around.
Aegislash: It is a good pokemon, but it is way overhyped. King's Shield is very predictable giving the other team a free turn to switch out or set up. I've heard talk about mixed Aegi but since I'm at the lower end of the ladder I haven't seen much of it (I need help team building >_<). But yea, having bulky ground types or normal types with a move to hit Aegislash hard is all you need to defeat him.
Underhyped: Talonflame. Check the sig. I was one of the many who were skeptical of that low base 81 attack and now look at it. There are 3 pokemon that appear to be full on stops to it and they are Heatran, Rotom-W, and Rotom-H. However, all of them have their disadvantages: none of them have reliable recovery, yet are counted on to check and/or counter multiple other threats, plus Rotom-Heat is weak to SR. Also, Heatran can't do shit to it unless it wants to sacrifice being useful by adding AncientPower to its moveset the same way it added Stone Edge to hold off Volcaronas last generation. People might be thinking that it's STAB's are both resisted by rock-types, but it also has Steel Wing that can be used on any set.
I ain't even done yet. Normally when something has a short list of counters, people simply resort to revenge killing a pokemon. The thing that pushes Talonflame over the top is the fact that it cannot be revenged outside of the rare ExtremeSpeed users. Gale Wings is what really puts a lot of stress on teambuilding because you need dedicated answers to Talon's STAB's. Man, fuck this thing. Plus, if you pair it with this mole right here you can assure its counters will die.
Excadrill- I feel like no one has really been giving Exca any mentions at all in these threads. Everyone has been raving about Thundurus, the Deo formes, and Genesect since they have been unbanned. Weather may have been nerfed but Mold Breaker helps Excadrill hit those levitaters that wouldn't care about it otherwise. That means it is a great teammate for Talonflame since it takes care of all 3 pokemon that stop Talonflame in its tracks for the most part with just one move. I just started using him because I needed something to keep hazards off the field and wasn't super weak to status and Excadrill has been doing an amazing job for me so far.
Klefki- This thing is incredibly annoying. Before I started using Excadrill, I really had to play around this thing. Prankster T-wave is a serious problem and it has such an amazing typing. Priority Spikes and Screens are another amazing utility along with the SwagKey set, which I have not faced yet thankfully. I forget who said it but apparently this thing has basically put TormenTran to sleep for good. The annoyance of this Pokemon is on another level.
EDIT@below: T-tar doesn't exactly appreciate a U-turn or Steel Wing. Especially a banded u-turn or a +2 SW.