Well to be frank, Blissey and some other choice Defensive Pokemon can't do anything back to it at all because of Magic Guard. Also, it has a pseudo-WoW with Flame Orb + Psycho Shift, which deters Physical attackers from switching in. I'd honestly say B is perfectly fine for it.Sigilyph... Eh. I have not seen it in a really long time (well except on the lower ladder where people still don't use it correctly...) but I still believe it has some utilities on certain teams due to its own versatility. If I had to make an educated guess here based on my own theorymon-ing here, which admittedly is shaky af due to my lack of knowledge on it, I'd say it belongs in B- rank.
Now I'm not sure exactly what its best set is, but I know it's pretty tough to put a stop to its Cosmic Power set (or even its CM set) once it accumulates several boosts (presumably 2/3+ CPs), and quite frankly, it has tons of opportunities to set up its boosts. However, with swift action, Sigilyph can be easily dispatched using a very strong attacker like LO/Specs Hydreigon or CB Victini (two common 'mons; mega Houndoom also completely fucks it over if no coverage outside of Stored Power), as with zero/one boosts, its defenses still aren't that great enough to withstand powerful SE hits.
Sigilyph also has the moveset to pull off a LO offensive set, but it faces a lot of competition from Specially based Victini in that compartment. Victini has the coverage to match/beat that of Sigilyph's and not to mention, the slight speed difference in this case is pretty fucking important. However, its ability to avoid recoil damage from LO as well as avoiding the 25% from rocks due to Magic Guard is enough to give its niche as an attacker, although its offensive sets are not really what I'd base its ranking off of.
Overall, I believe its boosting sets is enough to warrant a B- rank due to the great potential it has, but I'm not completely sure about it.
Oh, and I'll be on Showdown in like 18 hours. See you guys then.
Although CM/CP + Stored + Roost is fairly standard, the last slot allows for a bit of variation. It does have a large enough Special movepool to nail some of its checks. Ice Beam and Dazzling Gleam fucks up Hydreigon and the latter does the same to Houndoom. Air Slash is just good to hit stuff neutrally. Also, regarding Victini:
+3 4 SpA Sigilyph Stored Power (140 BP) vs. 0 HP / 4 SpD Victini: 192-227 (56.3 - 66.5%) -- guaranteed 2HKO
+2 4 SpA Sigilyph Air Slash vs. 0 HP / 4 SpD Victini: 166-196 (48.6 - 57.4%) -- 94.1% chance to 2HKO
Though Speed tiers is a completely different issue (thus Victini and a bunch of other Scarfers that run Knock Off). He's easily RKed by said threats without the Cosmic Power set. It's a strong sweeper/stallbreaker, but it's easily revenge killed. B is best for it.