: A -> A-
Gengar may be my absolute favorite Pokemon, but I can't deny his set of 3 flaws. The first is that pitiful bulk. Priority is Gengar's biggest problem and it usually tears him apart. Talonflame is the first to come to mind, as is Bisharp. Thankfully, this is somewhat remedied by Gengar's Ghost Typing, giving him an immunity to Mach Punch and Extreme Speed. The second is the Pursuit Weakness. Funny thing is, though, no Pursuit Trapper wants to directly switch into Gengar. Scizor is terrorized by Hidden Power Fire, while Bisharp and Tyranitar don't want to be around a Needs-More-Focus-to-Hit Blast. The final flaw is the speed. 110 is good, yes, but not as fast as other threats like Alakazam, Mega Lopunny, Mega Manectric, Weavile, Tornadus, Thundurus, etc. He still outspeeds Sweeper Garchomp (rare, but still used somehow) and Speed Ties with Latios, which shares similar flaws (bar the bulk, of course).
Now if we were judging Gengar solely as a Sweeper and nothing else, I would certainly agree with a drop. The problem is, though, is that Gengar is much like a roulette. You typically won't know know what he will use, bar Shadow Ball on a normal day. Is it a Life Orb Sweeper? SubDisable? Taunt+Will-o-Wisp? Destiny Bond? Hex? You just won't know until later. Gengar remains unpredictable in a meta where many can predict what a Pokemon will do; Garchomp is almost always TankChomp, Heatran is typically Stealth Rock and can pack Taunt or Earth Power, Charizard-X is Dragon Dancer, bulky Talonflame, Azumarill is either Choice Band or Belly Drum, Landorus-T is either bulky Stealth Rock setter or Scarf Revenge Killer, etc. There aren't many Pokemon that can keep opponents guessing (Mega Altaria can at first, for instance), so that's a bonus. Gengar also has a fantastic Special Attack that can push its way through opponents, so we can't discount that. Not to mention that while the Ghost-Poison typing + Levitate gives it immunities to Normal, Fighting, and Ground, but also a quad-resist to U-Turn, a commonly spammed move. It also gives him an immunity to Toxic, which is a nice little bonus, but it also gives him that Fairy and Grass resist (no, I wouldn't switch this into Celebi or Serperior, but still). Gengar can still switch into some moves like Earthquake, Close Combat/Drain Punch, Extreme Speed, etc, and I'll admit those moves are becoming more rare, but those are still valuable opportunities. Name a single Pokemon, bar Chansey or Hippowdon, that wants to switch into Gengar and have any remote chance of being unscathed. Considering that Gengar learns Shadow Ball, Focus Blast, Sludge Bomb/Wave, Thunder/bolt, and Hidden Power Fire, it makes switching into difficult until you figure out what it is. I can understand why people want it lower, but I still look at it and think "keep it A".
: A -> A-
I cannot completely comment on Mega Slowbro, but I can easily say that it seems...useless. I mean, it IS a tanking behemoth from the watery depths, but it loses what makes Slowbro nearly unbreakable: Regenerator. Sure an immunity to crits is nice, but those are rare anyways. Basic Slowbro can switch out and reheal itself to come back and wreck again. I personally see this Mega less viable than the base forme, but that's just me. I can see a drop, but I need more time.
: B+ -> A-
When I attempt to describe Togekiss, the first word I can think of is "Self Sufficient". In many cases, Togekiss can set-up on any Pokemon, bar Ground and Electric-Types, and proceed to be the most annoying thing ever with ParaFlinch (I used a Bulky Calm Set, personally--best 20 minutes of breeding yet). Of course, it's not just relegated to ParaFlinch; Togekiss has the ability to run Nasty Plot, Heal Bell, and even...Aura Sphere, a move that makes Tyranitar regret switching in. Something to remember is that there is a reason why Togekiss is Borderline. It's a monster dressed up as a Fairy. The only move I can consistently see on EVERY set is Air Slash (thank you Serene Grace). Outside of that, it's hard to tell what it will use. Most will use the Nasty Plot+Heal Bell set, but then there are those that run oddballs to throw people off. Don't forget that this is a tank with access to reliable recovery in Roost. I want to see Togekiss go up a rank. Me, personally, I see A- being the perfect home.
: A- -> B+
It feels like hours ago I was arguing about Shaymin--now it's the Gen II pixie. Celebi is a Pokemon that can sweep, Baton Pass, tank, etc. The Grass-Psychic typing gives it an edge in attacking and, oddly enough, a unique defensive edge as well. Sadly, U-Turn and Pokemon more effective against it are on the rise, so seeing Celebi drop...isn't surprising. I'm on the fence still, though. Like Mega Slowbro, I need more time.
: A- -> B+
I'm behind the times with Volcarona, as I use a surprising bulky set to throw people off, but I see why people would want a drop. Volcarona is difficult to get out on the field due to its massive Stealth Rock Weakness, it needs a turn to set-up before truly attacking, things are out that can stop it (see Azumarill and Manaphy), but...that's not all bad. Simply support can alleviate this annoyance and, honestly, Volcarona is a beast to use as far as set-up sweepers go. Quiver Dance is all like "I'm Calm Mind, but MOAR SPED", which is truly stupidly good. It's a shame that most Pokemon above, see Heatran, Charizard, Azumarill, Mega Altaria, etc. resist the STAB combo...and there's not much it can do for that besides run Hidden Power Ground for Heatran, which is the true claim to the possible drop outside of Stealth Rock. I prefer to see it A-; while it may be stopped by several key Pokemon, it also defeats other key Pokemon. At +1, even Alakazam doesn't want to come in on this, for the faster +1 Bug Buzz spells death. Scizor doesn't like that it has a Fire-STAB. Ferrothorn can't set Stealth Rock when it must combat against Fiery Dances, etc. Often underestimated, it still does well.
: B+ -> B
While I haven't really used Mega Gallade, why would I use it above other Fight-Types? Mega Medicham has Pure Power to give it unrivaled wall-breaking abilities, Mega Lopunny can hit Ghost-Types without a hitch while still being immune to Ghost-Types herself, Conkeldurr is bulkier, Lucario has Extreme Speed...I still don't see it. I'd like to see this drop--also Keldeo and Terrakion. Those two are too good.
: B+ -> B-
The best comparison I can think of is Excadrill, a Pokemon that requires weather to sweep. What are the differences? Swampert has better bulk and natural attack, hitting harder than Air Balloon Excadrill, and it has a single weakness. That's about it. Excadrill can hold Life Orb to make it more powerful, has Swords Dance to make it even easier to punch through opponents, has a better typing in many senses (resists to Fairy and immunity to Poison is nice), and can partner with better Pokemon to give it the weather it needs; Tyranitar and Hippowdon are just better than Politoed and Swampert running Rain Dance makes it miss on a coverage move, like Excadrill running Sandstorm. Surf's down with me.