Yeah, I definitely agree that Toxicroak should move up to
A-. It's a pretty good counter to the plentiful Water-types thanks to Dry Skin, and that gives it good opportunities to set up while your opponent switches out. It has a decent defensive typing, and it's offensive typing isn't too shabby either. I can see this frogger moving up.
I don't have much of an opinion on Mega Absol since I don't use it in all honesty. Seeing that it does have really good attacking stats, a pretty good offensive move pool and a great ability in Magic Bounce, I would think those tools are good enough for it to not drop down.
If I may make a nomination of my own, I was thinking of nominating
Barbaracle from Unranked to C Rank.
I think this mon is pretty underrated as a Shell Smash sweeper. If someone was to use a Shell Smash sweeper, Cloyster would come to mind because of it's move pool and great offensive typing, something that Barbaracle may kinda lack, but I believe Barbaracle has some useful pros over Cloyster.
While it's typing isn't that spectacular, it does have more resistances than Cloyster, being resistant to Flying, Ice, Poison, Normal, and very resistant to Fire moves. Because of these resistances, it is actually able to switch into some resisted attacks, something Cloyster isn't able to do much. Thanks to its decent 72/115/86 defenses, it can take some resisted hits pretty well, and can also take some neutral hits to. Rock/Water isn't a really bad offensive typing either, being able to hit many Pokemon in the tier for neutral damage, barring a few mons. While it doesn't have the greatest offensive move pool, what it does get are still decent coverage options like Cross Chop, Razor Shell, X-Scissor, Poison Jab, and the EdgeQuake combo, which all can do some pretty good damage coming from Barbaracle's 105 Attack.
It does have plenty of cons though. While it does have a few good resists, it's weaknesses are pretty nasty, being weak to the common Electric, Fighting, Ground, and especially Grass moves. These weaknesses can be alleviated somewhat by having team members that resist these moves, such as Salamence, Doublade, Chestnaught, etc. Depending on what play style you're facing off against, it might not have a good opportunity to set up Shell Smash. Some of it's best moves do have accuracy issues, which means if it doesn't hit the opponent, then it's probably good as dead. It is also slower than Cloyster, and because of that, it is outsped by more mons than Cloyster would be, even after a Shell Smash.
Despite these problems, I do believe Barbaracle has a niche as a Shell Smash sweeper, with its own differences and set of options to give it usage over Cloyster.
Edit: Here are some replays of Barbaracle in action.
http://replay.pokemonshowdown.com/uu-260611291
http://replay.pokemonshowdown.com/uu-260696335