I agree with this, SD Scizor works great, and running 252 speed EV's is completely viable to kill Heatran and so on. It can also check big metagame threats like Lati@s and Mega-Metagross, and pairs well with a lot of Pokemon as you mentioned because of the beastly typing. Mega Scizor is better overall because of higher speed + bulk, but base Scizor has a higher power output and of course doesn't take up your mega slot.I honestly think Scizor should move to B+. Despite of course being outclassed in every way by it's Mega Form, the support base Scizor can offer to other Megas through killing literally every fairy in the game is incredible. Things like Gallade, Gyarados, Slowbro and Lopunny really love those things out of the way and Banded Scizor cuts through them probably better than every other non-mega fairy check in the metagame. What I think really pushes it over, however, is an SD set.
Scizor @ Life Orb
Adamant
208 HP/252 Atk/48 Spd
-Swords Dance
-Bullet Punch
-Roost/Iron Head
-Knock Off/Bug Bite
This set allows it to beat every fairy in the game; in fact OHKOing every single one with Bullet Punch - including max HP/Dfc Altaria! - aside from Unaware Clefable and a few Azumarill. Clefable doesn't do anything unless carrying Flamethrower/Fire Blast however, which is rare on the unaware sets, and Azumarill is admittedly the bigger problem here, but with SR all of them are taken care of easily aside from AV. Talking of Azu, the speed EVs are to outspeed 164 speed adamant Azu; the fastest the common sets go.
+2 252+ Atk Life Orb Technician Scizor Bullet Punch vs. 240 HP / 0 Def Azumarill: 337-398 (84 - 99.2%) -- 75% chance to OHKO after Stealth Rock
This also allows Scizor to OHKO a ton of other common Pokémon, including but not limited to Lati@s - OHKOs bulky defog Latias with SR - T-tar, Gengar, Mega Gallade, and in general it's very hard to find something neutral to BP that isn't 2HKOed. Even the ones bulky enough to do so will not appreciate a Knock Off and most of the time can't outspeed.
I also think that Keldeo, although it is amazing by all means, doesn't really deserve S rank for me. It's quite easy to check, and despite burns from scald and so on there are several Pokemon that can switch in quite consistently.
Poliwrath seems interesting, but it is a hell of a lot easier to check than Keldeo. Even though it has Circle Throw I feel like it's so easy to take advantage of it because of all the things that switch in easily (unless I'm missing something - don't have a lot of experience against it). It's function is to phaze, but it doesn't threaten any prevalent Defogger.