Okay there was a lot of discussion earlier about this but I didn't see a clear conclusion so I'd like to renominate Sylveon for B-. Since I have very little experience with its cleric set and I feel that it has been discussed to death, I will not bring it up. However, I have been using the specs set quite a bit and can say that it is very underrated. With specs on, Sylveon hits incredibly hard 2HKOing pokemon like spDef gliscor, spDef Hippowdon, and mew( This guy is incredibly important because many people switch into sylveon with him thinking he's taunt bait). Also, even without SpDef investment, Sylveon has usable special bulk that grants him switch in opportunities.
Another important thing to consider is that there is very little competition for a fairy type spAtking wholepuncher. Fairy type is an incredible type right now in the metagame and specially offensive fairies are not very common in the metagame. In fact, his only true competition in this field is Mega Gardevoir, who while is better than sylv, takes up a mega spot that can be used for another mon on the team. Another pokemon people will bring up when discussing spAtking Fairies is Clefable. While clefable does somewhat offer this role, it requires some sort of setting up before it's hitting as hard as sylveon.
One last thing to consider is look at the other pokemon sylv is being ranked with right now. In my opinion, Sylv is so much better than Absol, Aggron, and every other mon in C+, while being as good if not better than B- mons like zapdos, weavile, and sableye. I personally would like it to go to B rank but i can settle for B- for now.
tl;dr Sylveon's spec set is truly underrated and is deserving of a higher viability ranking
Another important thing to consider is that there is very little competition for a fairy type spAtking wholepuncher. Fairy type is an incredible type right now in the metagame and specially offensive fairies are not very common in the metagame. In fact, his only true competition in this field is Mega Gardevoir, who while is better than sylv, takes up a mega spot that can be used for another mon on the team. Another pokemon people will bring up when discussing spAtking Fairies is Clefable. While clefable does somewhat offer this role, it requires some sort of setting up before it's hitting as hard as sylveon.
One last thing to consider is look at the other pokemon sylv is being ranked with right now. In my opinion, Sylv is so much better than Absol, Aggron, and every other mon in C+, while being as good if not better than B- mons like zapdos, weavile, and sableye. I personally would like it to go to B rank but i can settle for B- for now.
tl;dr Sylveon's spec set is truly underrated and is deserving of a higher viability ranking