(you can read the bolded parts and skip the nonbolded parts of this post for a very barebones summary)
I think that Jirachi is a very solid A+ tier pokemon, but it is definitely not worthy of the S rank.
When I think of Jirachi, I think of solid defensive typing, and bulk. 100 Spd is not enough to outpace the fastest, strongest threats, and 100 Atk or SAtk is pathetic in the OU meta, especially without any moves above 95 BP (STAB psychic is horrible) save Thunder in rain. Jirachi needs rain support to be an S tier threat. SubCM variants outside of rain suck because 95BP thunderbolt isn't going to be doing much damage coming off 100 attacking stats, even with a boost or two (or four). Rain takes away Jirachi's fire weakness and gives Jirachi the opportunity to abuse 120 BP Thunder with 60% paralysis chance, far more difficult to switch into than scald. The Sub CM set 6-0s many teams in Rain, while stall teams struggle to break Jirachi's subs even outside of it.
The Scarf set is very nice (and horrifically weak), and the ability to cripple walls with Trick is criminally underrated, while Healing Wish allows many teams to pull off unbelievable comebacks, but this Jirachi set is more of a supporter. It can outspeed +1/+1 Dragonite and severely dent it with Ice punch or Try to break Multiscale and Flinch it to death with Iron Head, but it's basically setup bait once it revenges a pokemon. Jirachi is just too damn weak and still, too slow to defeat the faster, more powerful pokemon of OU.
Specially Defensive Jirachi functions very well against Rain in that it resists Hurricane unlike Ferrothorn and Celebi who is 2x Weak, and can take Thunders fairly well resisted by both Celebi and Ferrothorn, but even it gets flattened by Rain Boosted Hydro Pumps, which are far more threatening. The resistance to Dragon type moves makes Jirachi very solid, but the Specially Defensive set is complete setup bait against anything with a sub, because this Jirachi set has no offensive presence(nastypass Sdef Celebi, Ferrothorn's 120-150BP STABS) and still, every Jirachi needs support from Latias, Gliscor, Lando-T, or Gyarados against power Earth and Fighting type physical moves that plague the tier. Given the offensive nature of this metagame, it is great that Jirachi resists Dragon; but all it can do is take a few attacks, then it dies due to poor recovery.
100/100/100 defenses are very solid and usually ensure that it will not be getting OHKOed, but all common Ground pokemon like Mamoswine, Garchomp, Donphan, Hippowdon, Landorus-T, and Gliscor all deal heavy damage to Jirachi, even without a boosting item!
252 Atk Garchomp Earthquake vs. 0 HP / 0 Def Jirachi: 326-386 (95.6 - 113.19%) -- 75% chance to OHKO
This calc is from scarfchomp, not band or LOchomp. Banded Dragonite, basically +1 Dragonite will also KO, not really a lot of power required to break through that typing. Banded Garchomp also KOs every single Jirachi set including 252/252+ Jirachi (20% of the time, 75% with an adamant nature)
Jirachi's typing, Psychic/Steel is a mixed blessing. The Psychic typing ensures that Jirachi is not weak to fighting attacks, (though it cannot handle more than one throughout a match), provides it with a 4x resistance to psyshock in calm mind wars, but also ensures that it takes neutral damage from the ever present U-turn. Unlike most other steel types, Jirachi has no notable resistances aside from Dragon moves, something every other steel type brings to the table. Jirachi doesn't help take water moves better like ferrothorn, fire/bug/dark moves like heatran, or ground moves like skarmory. Resisting Hurricanes or Ice beams and taking neutral damage from fighting attacks is something it has above Ferrothorn, but defensive pokemon thrive on 4x and 2x resistances and immunities, not neutralities. If Jirachi had reliable recovery in Recover instead of Wish, it would be much better, though still not an S-tier defensive threat, but unfortunately, it must use/waste two slots on Recovery in Wish and Protect. This does allow Jirachi to pass on 202 HP Wishes to its teammates, and play differently than other special walls.
Jirachi cannot take Hydro Pumps in Rain, or any fire attacks reliably in Sun
252+ SpA Choice Specs Politoed Hydro Pump vs. 252 HP / 236+ SpD Jirachi in rain: 274-324 (67.82 - 80.19%) -- guaranteed 2HKO
0 SpA Ninetales Flamethrower vs. 252 HP / 236+ SpD Jirachi in sun: 188-224 (46.53 - 55.44%) -- 15.23% chance to 2HKO
Jirachi is easily burned by the ever present Scald, and will not enjoy switching into it anymore than Ferrothorn does, but definitely much less because it does not resist water attacks. This cannot be said enough times. When the best Politoed set is Specs, and Jirachi is easily 2HKOed in its best weather, it's really not doing well as a defensive wall. One might think that Jirachi is not too badly off in OU, as there are only four fire types. I'm not going to talk about Infernape; not because it sucks (it fills niche roles in OU and can function quite effectively), but for simplicity. Each fire type in OU presents an insurmountable challenge for Jirachi. For starters, against all of them, Jirachi will have to Iron Head twice as often to flinch them to death, usually, never going to happen. Volcarona threatens to outspeed Jirachi and tanks any special attacks after a boost. Flame body provides a 30% chance Jirachi be burned and crippled for the rest of the match should Jirachi go for the Iron Head or Body Slam, while Lum Berry is common enough on Volcarona that a Thunderwave as The Fire Bug Quiver Dances means game. Jirachi has nothing that can KO the bug, and outside of Tricking a Scarf onto it as it Quiver Dances or Fire Blasts, then switching to a counter, cannot do anything substantial besides paralyze.
252+ SpA Choice Specs Too Weak Jirachi Psyshock vs. 0 HP / 0 Def Volcarona: 255-301 (81.99 - 96.78%) -- guaranteed 2HKO
Heatran, one of the most common pokemon in OU, even on Rain teams provides a solid 4x resist to Iron Head. The first set on site usually carries an Air Balloon which must be popped before Superachi can think about KOing it, which it will never do without a life orb. Jirachi is too weak to KO any threats with coverage options unless they are 4x weak. I wish I didn't have to bold this, but its typing sucks offensively.
Modest Jirachi fails to KO heatran with HP Ground without a Life Orb.
252+ SpA Life Orb Weakrachi Hidden Power Ground vs. 0 HP / 4 SpD Heatran: 348-411 (107.73 - 127.24%) -- guaranteed OHKO
If Heatran is even remotely bulky,
252+ SpA Life Orb Jirachi Hidden Power Ground vs. 248 HP / 252+ SpD Heatran: 255-302 (66.23 - 78.44%) -- guaranteed 2HKO
0 SpA Heatran Lava Plume vs. 4 HP / 0 SpD Jirachi: 218-258 (63.74 - 75.43%) -- guaranteed 2HKO, guaranteed OHKO in Sun
I've already presented a calc from the weakest of Ninetales, and the Most specially defensive of Jirachi, and it doesn't look pretty. Most Chlorophyll sweepers run HP Fire due to PseudoSTAB which seriously dents Jirachi. Ninetales makes things worse for Jirachi, as it brings up pokemon like Victini, Darmanitan, Infernape, and CharizOrd from UU as viable OU pokemon on a Sun Team, which frequently run Donphan and Dugtrio. Jirachi is dead weight against Sun teams.
Specially Defensive Jirachi is an annoying support mon that flinchhaxes and paralyzes opposing pokemon, and it cannot be pursuit trapped. Unlike Latias, it doesn't care about Stealth Rocks or Toxic, but that extra 30 Special Defense, and that additional 10 speed makes a world of difference in OU. However, Jirachi is very vulnerable to Spikes, Magnezone, and Dugtrio, all of which cause it severe problems.
Now, it's quite clear that every single Jirachi set bar Shuca Berry Jirachi is destroyed by Garchomp or any reasonably powerful Earthquake or Fire Blast. I imagine many of you saying or yelling, "what about Shuca Berry Icy Wind Jirachi?" Sure, maybe once in a while you will actually surprise your opponent and take down that Garchomp, not even in one hit, but in two(you can just switch out); the second after lowering said Garchomp's speed (but still not if the Garchomp is ScarfChomp). Lure Jirachi might work in some cases and net you a few kills against some of the most experienced players in OU, but once the surprise is out (it's pretty obvious when you send in a Jirachi in on my Garchomp) you're left with a substandard Jirachi set which at best is a lure for one pokemon (using HP grass for gastrodon, a UU threat? Wonderful. How does that stack against my DragMag HO team?) and at worst has terrible coverage against the OU metagame. The lure set is a gimmick, and in most cases you're left with an inferior, outclassed pokemon in a tier where each pokemon matters tremendously.
The defensive sets have no offensive presence, the offensive sets have no defensive presence, while the other sets purely play a supporting role. The SubCM set under rain provides the best of both worlds, though even it will fall to physical attacks and status. Jirachi's best traits are its sheer versatility and unpredictability, it's ability to annoy all pokemon with flinch and paralysis, and its resistance to the powerful Dragon type. However, Jirachi's shortcomings lie in its average 100 Base Stats, ensuring that aside from annoying, it will merely be average at whatever it does, be it scarfer or special wall, and never a premiere defensive threat due to its weakness to Rain Boosted attacks, its mediocre recovery in Wish and Protect, and weakness to Earthquake, common on many offensive teams. The SubCM set is capable of turning Jirachi into a premiere offensive threat, easily sweeping the majority of the metagame, but it is somewhat dependent on Rain for a powerful Thunder/Paralysis chance and resistance to Fire attacks. Jirachi is not fast enough to outspeed OU's strongest pokemon, and even if it could, it lacks the brute force and powerful STABs to tear apart the OU metagame. Jirachi is like Infernape in that one of its major weaknesses is taken away in its select weather, and that it shines. However, in the opposing weather, Rain for Infernape, and Sun for Jirachi, it becomes dead weight, worse than useless against an entire team style.
I think that Jirachi is a solid pokemon and could be slapped onto many teams with great success, but its shortcomings, lack of solid STAB moves, weaknesses, and need for support make it an A+ pokemon at best. Jirachi's support movepool is very solid, and it is one of the best A+ pokemon without a doubt, but it definitely does not belong in the S rank.
edit: (spelling)
