I'd like to nominate Mandibuzz from Unranked> D
On the surface, this thing might seem like a washed up version of Yveltal. And to an extent, you wouldn't be wrong to say this, Yveltal can do nearly everything that Mandibuzz can do better, except for 1 thing. Mandibuzz has access to Defog, whereas Yveltal does not. Unlike fellow Defog brethren in Lati@s, Giratina(-O), and some support forms of Arceus, Mandibuzz fits perfectly onto bulky/stall teams and has about everything they could want. It has reliable recovery in Roost, it can deter some setup sweepers lacking Rest/Refresh with Toxic, and my favorite part, is that it can check prominent Pokemon like E Killer Arceus, P-Don, and Mega Mence. Unlike Yveltal, though, Mandibuzz can check the aforementioned threats (provided E Killer isn't running Stone Edge) in addition to being able to check other threats, like Mega Mewtwo Y, Lugia, and Giratina-O since it can viably run a mixed bulk set. I've provided some calcs and replays below.
http://replay.pokemonshowdown.com/ubers-437512521
http://replay.pokemonshowdown.com/ubers-400471869 (This one is old but still relevant)
To highlight what it can do to combat offense:
+2 0- Atk Mandibuzz Foul Play vs. 4 HP / 0 Def Arceus: 252-297 (65.9 - 77.7%) -- guaranteed 2HKO (Don't want to setup Swords Dance right in this thing's face)
+1 0- Atk Mandibuzz Foul Play vs. 80 HP / 0 Def Mega Salamence: 220-261 (62.6 - 74.3%) -- guaranteed 2HKO (This right after it sets up Dragon Dance)
0- Atk Mandibuzz Foul Play vs. 0 HP / 4 Def Mega Mewtwo Y: 350-414 (99.1 - 117.2%) -- 93.8% chance to OHKO
0- Atk Mandibuzz Foul Play vs. 0 HP / 4 Def Arceus-Ghost: 252-296 (66.1 - 77.6%) -- guaranteed 2HKO
+2 0- Atk Mandibuzz Foul Play vs. 0 HP / 4 Def Arceus-Ghost: 500-590 (131.2 - 154.8%) -- guaranteed OHKO
0- Atk Mandibuzz Foul Play vs. 0 HP / 4 Def Mega Gengar: 156-186 (59.7 - 71.2%) -- guaranteed 2HKO
Now to highlight what it can do on the defensive end. This is assuming I have 248 EVs into HP, 136 EVs into defense, and 124 into Spec D
+2 252 Atk Spooky Plate Arceus-Ghost Shadow Force vs. 248 HP / 136+ Def Mandibuzz: 170-201 (40.1 - 47.5%) -- guaranteed 3HKO after Leftovers recovery (That's after a Swords Dance boost, meaning Mandibuzz can easily switch into this and OHKO it with Foul Play effortlessly)
+2 252 Atk Life Orb Arceus Extreme Speed vs. 248 HP / 136+ Def Mandibuzz: 246-290 (58.1 - 68.5%) -- guaranteed 2HKO after Leftovers recovery
252 SpA Mega Mewtwo Y Ice Beam vs. 248 HP / 124 SpD Mandibuzz: 246-290 (58.1 - 68.5%) -- guaranteed 2HKO after Leftovers recovery
244+ SpA Griseous Orb Giratina-O Draco Meteor vs. 248 HP / 124 SpD Mandibuzz: 241-285 (56.9 - 67.3%) -- guaranteed 2HKO after Leftovers recovery (Does not factor the Spec A drop. The 2nd one won't KO without having had prior damage on me first, meaning I can simply get a read on it, Roost off the damage, and wall this thing to hell and back, or at the very least, force a switch and punish the switch in with Toxic)
248+ Atk Griseous Orb Giratina-O Shadow Force vs. 248 HP / 136+ Def Mandibuzz: 93-110 (21.9 - 26%) -- possible 5HKO after Leftovers recovery (Can easily switch into this with no fear whatsoever)
252 SpA Mega Gengar Sludge Wave vs. 248 HP / 124 SpD Mandibuzz: 175-207 (41.3 - 48.9%) -- guaranteed 3HKO after Leftovers recovery
Now, as deterrent to setup sweepers and frail Pokemon, I understand that Mandibuzz is unfortunately outclassed by Yveltal. However, having access to Defog gives Mandibuzz slightly more utility than Yveltal does, and makes it a better fit on stall teams than Yveltal. Ii mean, at the very least, Mandibuzz has got to be more viable than useless Reshiram, Fire/Psychic Arceus, and Deoxys-N. Come on now. I think the information I've provided can attest to at least that.