Agree w/ the above stuff...at least about Muk. It's a good Poke, but it's moves have low BP, and all its stats are fairly mediocre(bar speed which just sucks, but that's why it's a Curse.) I definitely thought about using Minimize w/ Curse, especially as Clefable is beat by Muk, but I'm not sure what Muk'd go w/ for mono type coverage. Definitely not Poson Jab. Speaking of PJ, I actually didn't recommend it alongside Curse and Rest because it provides bad coverage, and Muk can Poson(albeit not badly) w/ Poison a Touch. I think C- is about what I was no mind it for, so I'd be glad to see it go there.
Azelf is kind of one-dimensional, but I'd argue it doesn't have to be a lead. It's fast w/ great attacking stats, so it can make use of Trick w/ a Choice Scarf, or a mixed attacking set, maybe w/ substitute. Still, it's probably about a D anyways, just cause it's bulk is so lacking, and it's typing doesn't help it at all.
Uxie seems like it should be in the running for a decent rank. It's a little less bulky(75 base HP), but it's faster w/ access to Foul Play, Memento, and even Giga Drain, which might be nice w/ Calm Mind. U-Turn is pretty nice, too.
Mesprit is..probably not worth ranking, especially if we rank the other pixies. Healing Wish is nice, but Cresselia completely outclasses it in that role.
Muk
Damage isn't that great; this is correct. Bulk isn't bad overall, especially on the Special side (it's well above Char-Y for example, and Char-Y is pretty nice on natural Special bulk), and speed is in the shit level of Conkeldurr.
Coverage, we basically have Poison Jab + Shadow Sneak/Punch + Power-Up Punch + Elemental punches; and with Poison Touch, I may add, that's an additional +30% Poison chance on every contact move Muk uses (it does not affect the foe using a contact move; ie Khan using Return on Muk will never be poisoned), so Poison Jab basically doubles in Poison chance, PuP is a +1 atk boost PLUS a 30% poison chance for free residual damage. Standard Poison status isn't the best status in the game (imo, the worst due to almost no advantage over Burn) but a free 1/8th max HP per turn residual damage adds up fast, and Muk can spread it like ebola. Coverage, Muk always stinks (punny) unless it's running a CB or AV set. Which it generally should not run lol.
Very scary answer to Fairy types, though Gardevoir is a bitch. Deals with Fighting mons fairy well despite being glorious Guts bait, as most don't run Psychic or Ground coverage. I like it, personally, but it usually is running a setup set (PuP + Stockpile being my personal reccommendation) that isn't that great largely due to the Ground weakness. C- sounds fine. It's one of the better Poison types available, imo, but not that grand.
Uxie
One simply cannot look at Uxie without its same-gen bulky, offensively deadweight rival in Cresselia. Identical offenses, Uxie has a smidgen of speed, but a grotesque 45 point gap between Uxie and Cresselia's HP is the primary reason you very rarely see Uxie (and when you do; it's usually a noob with no idea wtf he's doing). So, we basically have to differenciate them by movepool (and the fact that all 3 pixies are cute, so bonus points). Uxie, first set that comes to mind is the dual-screens setup set (Memento + Screens + U-Turn); much like Latios, just a bit slower for more bulk and U-Turn so you don't HAVE to sac your setup mon. Cress could do the same with Lunar Dance I guess, but the point is to set up on something utterly crippled, instead of a heal+switch into a probably fully healed mon against a non-crippled foe. imo that's Uxie's most viable use is a brutal setup sweep with Dnite / Mega Luke / SD Chompy / PuP Khan / whatever other checkmate sweeper you like. In terms of other options, Uxie has Knock Off, Stealth Rock, Yawn, U-Turn, Foul Play, Heal Bell, and Trick is most of what stands out to me. Uxie, tragically, lacks reliable healing and Ice Beam (Ice Punch simply doesn't cover the same theats), and the superior bulk that Cress has is very insturmental to its effectiveness. imo, no higher than B- on Uxie: Cress is A, Latias is yet another similar mon, and Uxie is mostly outclassed by those two. As for the cripler set of Screens+Memento; Latios is pretty damn good at that one and the higher Speed helps cover the bulk gap by screening BEFORE getting hit. PLus, recover/roost. So yeah, B- on Uxie. Imo.
Azelf
Lead set is infamous since what, D/P? I guess you really, really depend on Stealth Rock so much it's worth sacrificing an entire team slot just to set it up once asap, the Azelf is great at Taunting, clicking SR, and then Exploding after the Sash is broke in front of a Garchomp that might just Dragon Claw you twice as you Taunt -> Stealth Rock and get a kill for almost nothing.
Azelf is, imo, similar to Alakazam: fast, strong, frail Psychic mon. Bit slower, bit weaker, quite a bit bulkier but still frail, but also has a movepool. You know, Thunderbolt, Fire Blast, U-Turn, Nasty plot, all kinds of stuff Zammy never did get.
Mixed sets have Explosion, Ice Punch (slightly better than HP Ice I guess) and a better Knock Off.
Pretty scary after a Nasty Plot boost, can lay down some pain, and I guess run a suicide SR set. Lot like Alakazam, just has Levitate for switch in opportunity and actually has coverage. imo, C+, it's comparable to Starmie.
Mesprit
So much hate wtf.
Well balanced stats let it takes pretty good hits, offenses hit harder than you'd think, and its support movepool is good too. I ran it in Doubles not long ago (different meta, I know, but similar concept here) to counter stuff like Garchomp, Mence, Gyarados, Terrakion, Mega Venusaur, etc. by running it sort of like Porygon2: middling speed, good coverage in BoltBeam, good power, and good bulk. It's a tank. It's not an offensievly dead Uxie or cresselia; it's not an "Oh fuck they sneezed at me D:" Weavile or Alakazam. It's bulky and strong, not tough or brutal. Scarf lets it take hits, hit pretty decent between Psychic/U-Turn or Trick/Ice Beam/Thunderbolt, and revenge kill. Expert Belt lets it take hits, switch moves, and nail things harder; more for OHKOs than 2HKOs. Healing Wish lets it heal a teamate before going down, like cress, but offers a lot more power instead of bulk. Also has Stealth Rock, and similar support to Uxie, again, in exchange for power>bulk. It's niche, and it should be ranked the lowest of the pixies, but imo it's not that bad. Comparisions would be more along the lines of things like Special Swampert (who is running overall worse stats, I may add), Conkeldurr, and other tanky mons.
Lack of Fire coverage isn't too harsh; Ebelt HP Fire handles Scizor/Ferro if you need that, Fire Punch works too if you don't want to lose IVs but needs sun or chip damage. Personal favorite pixie, but again, should be ranked under Uxie/Azelf so I guess C or C-. If ranked. I'm the only non-retarded person that would run one anyway.
edit: ok maybe I'm a bit high on the ranks, blame the vodka k. Can throw Azelf/Uxie lower, like C/D instead of B-/C+, idc.