Spectoed is actually a surprisingly good wallbreaker. It can almost ohko Ferrothorn, which it often finds itself up against. If it runs hp grass, it can 2hko Jellicent, and its Hydro Pump 3hkos Blissey, so it's important to have a bit of damage of Blissey already, maybe by u-turning to it first. Gliscor goes down to Hydro Pump or Ice Beam, and naturally wall-ish offensive Pokemon like Jirachi can't take a Hyrdro Pump either. And obviously it takes Skarmory down. A special water/fighting/grass moveset with good power behind it threatens most of the walls in OU.
Isn't that set the standard? Politoed is so strong with Specs that it can defeat things that resist its attacks, much like Keldeo, although Politoed doesn't have nearly as much power as Keldeo. I prefer the defensive sets, though, since they have much more survivability.
I'll be adding spoiler tags for each section for the best and most innovative sets. Terrakion definitely belongs there I'll agree, even if I don't like CB terra because you can't run his double dance set.
Anyway Sub CM Jirachi is a pretty cool 'mon that I've experimented with on my rain team. Sub and its awesome typing + bulk makes it pretty hard to take care of with attacks found on defensive pokemon outside of ground moves and calm mind is self explanatory. After two boosts which isn't actually that hard you do actually decent damage with the unSTAB attacks you carry and become even bulkier specially then you are physically.
Where Jirachi is completely unique is the fact that its moves have a 40% chance to confuse and a 60% chance to paralyse. Assuming you get both affects off on a wall, even if you're doing pitiful damage because they resist both, they have only a 37.5% chance to act at all on that turn. Those kinds of odds combined with your own bulk renders you nearly untouchable. You're immune to sandstorm, resist rocks, can't be poisoned and have actually competent offences. Jirachi breaks down stall through sheer hax, and if it carries its sub over after a wall goes down it can start messing up offensive pokemon the same way.
Jirachi @ Leftovers
serene grace
bold nature
252hp/252def/4spdef
-thunder
-water pulse
-substitute
-calm mind
This set can also be used outside rain teams. I've used SubCM Jirachi a lot on a sandstorm team, and it's an incredibly effective wallbreaker/stallbreaker. If you aren't using Water Pulse, the only thing that you need to remove are Gliscor and Hippowdon, (and Quagsire of course) and on my opinion they are easy to remove or cripple pairing Jirachi with something like Celebi, Gothitelle or Keldeo. Once Ground-types are out of equation, there aren't much walls capable of stopping Jirachi. Jirachi can even use many common threats on stall teams, like Jellicent lacking Taunt, Tentacruel, Forretress, Ferrothorn, Chansey and Blissey, as setup fodder. Against offensive teams, Jirachi can sweep late game, or if you have paralyze support, mid-game at best.
The moves that I generally like to use the most are Thunderbolt and Psyshock. Psyshock because it gives a way to bypass the pink blobs without having to boost all way to +6. It also gives Jirachi a chance to win Calm Mind wars, as thanks to Jirachi's double resistance to Psychic, even a +6 Psyshock does little damage to Jirachi, even without any investiment on physical Defense, and against Keldeo, Jirachi generally wins because Keldeo is weak to Psyshock. Flash Cannon gives Jirachi a way to defeat Celebi, Tyranitar (without Water Pulse or a +6 Thunderbolt), and physically defensive Ground-types.
Yes, I know that Jirachi can only hax the opponent using Thunder and Water Pulse. However, these moves are only good on Rain, and if I want to use Jirachi on other weathers (or non-weather), Psyshock, Flash Cannon and Thunderbolt are solid alternatives that don't have the same hax level as Thunder and Water Pulse, but are generally stronger and don't detract Jirachi from its main job, that is wallbreaking/stallbreaking.
OBS: I prefer to use a bulky speedy EV spread of 252 HP / 4 SpA / 252 Spe instead of a physically defensive, on more offensive teams, as the speed is very good to outpace unboosted things like non-Scarf Genesect, non-Scarf Haxorus, Dragonite and to speed tie with non-Scarf Salamence, Celebi, Victini and some other things. While Jirachi's speed is definitively average compared to the speed demons that we have on the metagame, more speed allows Jirachi to sweep late-game once you remove anything faster that can trouble Jirachi. Not to mention how good is to outpace Tentacruel and Jellicent on stall teams.