Alright since the topic is stuck on freaking espeon I'm just going ahead and suggest some fresh nominations:
Mega Garchomp B --> B+
Togekiss B+ --> A-
Mamoswine B+ --> B
Mega Gallade B+ --> B
1) Mega Garchomp B --> B+
Until recently, I have thought that this thing is inferior to most wallbreakers, takes a mega slot, and just need a lot of support to function correctly. However, after using it recently to an extent of success, I can easily say that Mega Chomp is on par with Mega Heracross, Victini, and Diggersby in terms of viability and, particularly, wallbreaking ability.
First, it's speed isn't as bad as everyone thinks, it has better speed than Hoopa-U, Mega Heracross, and many other mons who are the premier or best wallbreakers in this teir. The fastest wallbreaker is probably Mega Medicham, but after that is Mega Garchomp. It outspeeds Adamant Excadrill with a neutral nature and Hoopa-U and Jolly Excadrill with a positive nature.
Now, lets compare Mega Garchomp to the 2 best wallbreakers in the tier, Mega Medicham and Heracross.
Mega Medicham: Faster than both, can 2HKO Phys. Clef without support (Sand for Mega Chomp). Has priority, deals good with HO, kinda bad vs. stall.
Mega Heracross: Can break Slowbro much better than both. Can break through Mega Sableye at +2 unlike mega medicham. Deals horrible vs. HO, good vs. Balanced and Stall.
Mega Garchomp: Best defensive typing (proven by tank chomp), biggest bulk, better speed than Mega Heracross, and can take down Skarmory risk-free (heracross's 2hko is a roll and can die to BB, medicham has to risk HJK (only 10%) and BB does like 85% min.). Can take down Mega Sableye at +2. Overall deals decent vs. HO, v. good vs. Balanced, good vs. Stall.
Overall, obviously i was slightly overselling Mega Chomp, Mega Medicham is clearly the best wallbreaker overall, but I'd honestly say Mega Garchomp is as good as Mega Heracross (B+), if not slightly better. [Sand is not a necessary support for Mega Chomp btw, but sure it helps doing more immediate damage on bulky teams, only important 2hko it gives him is on Unaware clef]
http://replay.pokemonshowdown.com/ou-273776498 - This is a replay where a standard stall team which Mega Garchomp just destroyed late game, skip to turn 27 if you don't want to see boring clef vs. gothitelle.
2)
Togekiss B+ --> A-
Togekiss is without a doubt, one of the best stallbreakers in the tier. Without Goth, any stall gets pretty much 6-0'd, and if you manage to put goth out of the way, Togekiss wins. Not to mention it's also a very good wallbreaker, a very good Defog user, and a very good cleric w/ Thunderwave. Thank to its fantastic typing and solid movepool.
fleggumfl also nominated this yesterday-ish.
3)
Mamoswine B+ --> B
I don't think Mamoswine has the same viability as Diggersby, Breloom, and definitely not Suicune, which are all B+. Mamoswine is outclassed in every sense of the word by Weavile, and I know Weavile is A+, but they should be at least 3 ranks apart. Aside from that, Breloom is much better because it has access to both Spore and Swords Dance, and a priority a bit more spammable. Breloom outclasses it even as a Sash Lead although Mamoswine has rocks and endeavor, it's going to give up on either Icicle Crash/Spear, Earthquake, or Ice Shard for those moves, while Breloom has Spore, and punches holes in the opponent's through Rock Tomb/SD even as a lead. Diggersby hits harder AND has SD, albeit a bit less Speed. Suicune is just better over all, Pressure, Calm Mind, Scald, Roar, and all the defensive utility it brings is just amazing and its viability is no way on par with Mamoswine. However, it definitely has its niches, probably the most viable is the sash lead. It definitely is good alongside Hydreigon / Omastar under Rain / Scizor / Zapdos / Chansey, which are all B, in my opinion.
4)
Mega Gallade B+ --> B
Tbh idk why this was alongside the likes of Mega Heracorss, which outclass it in wallbreaking. Although, Mega Gallade isn't too bad, a nice 110 speed is its best selling point, but tbh I feel like Terrakion, at a close 108 Spe, outclasses it. Mainly because Terrakion resists Knock Off / Sucker Punch and gets a boost out of it, doesn't waste a mega slot, and also because Terrakion can run a Choice Band and hit like truck, or run Choice Scarf and potentially sweep late-game.
fleggumfl also nominated this to drop down yesterday and I'm agreeing with him.