Mega Gengar
130 Speed and 175 Sp. Atk, could, god forbid, be used to outrun and attack things, but that is what non-mega Gengar's lesser stats are used for.
Ranking Mega Gengar is basically just ranking PerishTrap since it's pretty much interchangable. It's vulnerable to things like Hydreigon, Mega Metagross (since PerishTrap Gengar, you know, doesn't bother to outrun and OHKO with Shadow Ball), Landog, Aegislash, Trick Room, Thundurus, and even it's own non-Mega to an extent, but Perish Song is so absurdly fucking broken when you're unable to switch that these faults mean almost nothing. 130 Speed, Protect/Sub, and Disable means that most stuff that can hit it, such as Landog, have only 1 easily Disabled attack to use. Mega Gengar being A is fine, it can't just be splashed onto nearly any team and kill 1-2 mons a match like Khan or Landog but I'm surprised to see it lower than Char-Y. Char-Y is amazing but it can't just kill literally the entire relevent metagame with cancer- I mean Perish Song. I know I passionately hate it beyond measure (surpassing even Chansey on my small list of hatred) but I do stand that it's virtually uncounterable except by extreme means that no team should have to stoop to (aka, Soundproof, Max Speed Electrode with Mental Herb, Taunt, and Volt Switch). I think people want to drop it lower because it only does one thing or something, but the one thing it actually does is just so atrocious that it should be A-, minimum. If you aren't prepared for it, you WILL die.
Hell, if you are prepared for it, you'll probably die anyway. But at least you have might have a chance. It's not super popular but it's a threat we should always bear in mind, imo.
Azumarill
Great typing, offensively and defensively, great power, and the bulk isn't too bad. May sound odd, but I view Azumarill similar to Machamp: strong, hard to OHKO, and tricky to deal with, but slow, and a 2HKO isn't unreasonable.
imo Azumarill is a bit slow for Tailwind and Icy Wind based teams, but perfectly fine for Thunder Wave and Trick Room based support. But, thanks to that typing and bulk leaving little to be desired, it can function on a last-attack basis and Aqua Jet isn't too shabby on priority. I personally have not had much issue with it; but I know when to respect a foe. It crushes a lot of relevent threats, including but not limited to, Landog, Hydreigon, Heatran, TTar, Excadrill, Terrakion, etc.
It can hit almost anything pretty hard, and knock Off / Aqua Jet even offer some utility. Fair bit of versatility: AV, Belly Drum + Sitrus, Lum Berry, Choice Band, and even the niche but still horrifying Perish Song variant. It's also one of four viable physical Fairies (Mega Mawile and Mega Altaria eat a Mega, and Granbull, but I'll save you all a wall and not discuss Granbull in this post) and all things considered it's pretty solid on a lot of teams; even alongside Char-Y.
Has trouble against faster, hard hitters like Mega Khan and Mega Salamence, but even in troubled situations it's not exactly dead weight. Partners well with Amoonguss as stated above.
imo, really solid mon that has shown success, A- is my vote. Doomed against Amoonguss / Ferrothorn / Mega Venusaur / Char-Y but no Pokemon is perfect.
Dewgong
It's adorable. A+ or bust.
Fake Out + Icy Wind was a very important niche in 2014 but much less relevent in 2015; better off with Blastoise, Weavile, etc.
Also has the rare Encore + Disable but derpy Speed prevents that from shining; use Whimsicott/Liepard + Mega Gengar for auto struggles, or Alakazam if you just want to save a team slot.
Pretty nice offensive movepool despite lacking Scald (the second biggest curse of all Ice/Water types), as it has Ice Shard, Aqua Jet, Perish Song, GROWL, Icicle Spear, Brine, Sheer Cold, and Drill Run.
Just leave it unranked lol.
Vivillion
Actually no, not going there.
Powder is an amazing move though guys, it's up there with Soak on the "Horrifying in ways few will ever understand" scale. Do remember it's +1 priority.
tl;dr:
Mega Gengar can stay in A or go down to A-, imo Azumarill from B+ to A-, Dewgong to A+ for being adorable.