So far it has to be tyrantrum looks cool decent move pool and can be a slow attacker (or boost from choice scarf/rock polish) or being support (stealth rock + dragon tail). What leaves me disappointed is its ability with no second choice and I was thinking they'd give him elemental fang but no dice so it has one usable move (two technically with bite).
If you mean letdown on a certain set, then Aegislash. His "standard" Swords Dance+King's Shield+Shadow Claw/Sneak+Sacred Sword set is way too predictable and falls very easily against prepared teams. Furfrou and Pyroar can check it, even Diggersby of all things checks it.
Keep in mind that this is only standard set though, apparently there's special sets out there that use Autotomize.
Togekiss, I worked so hard to make sure my XD Togetic had the perfect nature and ability that it could make full use of stab Tri-Attack in Gen IV and V then they went ahead and changed it's god damn typing.