Scyther has Swords Dance and usually has exactly one turn to set it up before needing to Endure to 1 HP (only Thunders from the Electrics and STAB/Marowak Rock Slides OHKO). Its +2 Reversal is significantly stronger than Heracross's STAB Reversal (guaranteeing the OHKO on Steelix and doing a lot more to Suicune/Skarmory), and its +2 HP Bug is slightly stronger than Heracross' Megahorn (and more accurate). It's also significantly faster than Heracross, outspeeding Suicune (which is decent) and Miltank (which is HUGE) as well as a bunch of stuff that doesn't matter. And it's immune to Spikes, which is rather nice. I have suffered full sweeps at the hands of SD/EndRev Scyther before; it has a niche. It's not usually worth the trouble, so it should be D Rank, but it does have a tiny niche and should be on the list.
To restate, I am referring specifically to Scyther with the set Swords Dance/Endure/Reversal/HP Bug. Its niche is that a) no other Pokemon (besides Scizor and Smeargle) receives both Swords Dance and Reversal, b) it is the fastest Reversal user, c) it is immune to Spikes - more important than usual, for something that likes being at 1 HP - which is also rare among Reversal users, the only others being Gyarados and the awful Yanma.