I think Kyurem although that one is questionable because it has been both over and underrated but in the end most people way underrate it it seems like. Considering it is tied with the Strongest user of Stab Draco Meteor (Latios), it is surprisingly not considered. Most people are turned off the whole Ice thing but Kyurem really is surprisingly bulky as well as a dangerous special attacker. Stab on both Ice and Dragon is pretty cool as well and while weakness to Stealth Rock is never a plus, people treat it like instant death to it which it most certainly isn't. With Wish and Spin support, it is pretty hard to kill and can set up Dual Screens and you still have to watch out for those blasted Draco Meteors which are a pain for most pokemon to switch in on. It has Focus Blast and Hp Fire for steels (although would have loved Flamethrower certainly as does Latios) and although 70% accurate moves are never stable to rely upon, it helps for coverage and other pokemon run it too (the reason why I mention this is that some people felt the need to penalize Kyurem for having to run Focus Blast and Hp Fire when so many other pokemon have to as well which is double standarding) and really helps killing Heatran and Nattorei (although Icebeam does hit neutral and could 2 hit ko weakened Nattorei's with Specs or Life Orb). Screen support is helpful as well and for being one of those awesome underutilized pokes that destroy Rain and water pokes especially. But the only real pokemon Kyurem fears are Specs Draco Meteors (Latios), Choice Band Scizor (handled by Hp Fire and defensive Kyurem carrying Reflect), Breloom if Kyurem's weakened, and steels almost never seen nowadays Jirachi/Metagross, and Brongzong. Short of that, very little can switch in safely and Dragons fear Kyurem's Ice and Dragon moves. I feel kyurem is underappreciated.
Slowbro is another good option for Regeneration and the importance of having a Fighting resist as well as a 50% recovery move, status options, being able to set up sweeps or sweep itself with Calm Mind and Trick Room, etc.
Zoroark as people have mentioned. It's weak defensively but it's fast and is so fun for mind games (especially without Team Preview but with could cause some pretty fun mindgames as well) and is a very strong attacker. There's nothing more fun than messing with people (and if you see Haxorus, there must be a Zoroark somewhere since Nattorei has such a bad tendency of switching in on it so knowing that, the opponent could be very hesistant of switching in since all of the priority and steel users wouldn't like the chance of switching in on Zoroark). Zoroark has lots of fun things to pretend being like Infernape, Blaziken, Haxorus, Gengar, Mismagius, almost any fighter (barring Stealth Rock), Heracross, etc. It also pokes holes in the opponents team and could remove vital counters to allow something else to hit through which is awesome.