I don't particularly care about C rank and below, but here are a few mons that are at least somewhat viable (some more viable than others) and probably deserve to be moved out of E rank into the realms of visibility (plenty of them above C rank, at that):
Niche FM/RP users
Tangela
Electivire
Pachirisu
Magmar
Non-Mega Blastoise
...which got me thinking about other Non-Megas that are definitely viable but only the Mega is ranked
Non-Mega Latias
Non-Mega Manectric
Non-Mega Diancie
Non-Mega Gengar
Non-Mega Scizor
And a few other random ones:
Deoxys-S
Crobat
Rhydon
Raichu
I reckon most of the FM/RP users would go in D rank because they're viable but barely so and that's due to a small niche, possibly in C somewhere. Pachirisu deserves E rank cause it's outclassed by Electivtire (has better bulk and offence, and can afford to tweak its moveset where as Pachirisu will always have the same 4 moves).
In the past I was against ranking Megas/non-megas separately, but now I'm for it. It's standard across the site which makes it easier for people who are already familar with Smogon, and it does give more info about the Pokemon.
Anyways
Rhydon for C+ rank
It's a better Lightning Rod user than Rhyperior as it has better bulk thanks to Eviolite, even better than Assualt Vest Lightning Rod Rhyperior (and also gets Protect which is huge when you have 2 4x weaknesses and stay alive to prolong Lightning Rod support).
FYI I use 168 HP / 176 Atk / 164 SpD. For a demonstration of its bulk, 168 HP EVs lets Rhydon avoid a 2HKO from Landorus-T's Earthquake 99.6% of the time. In return, 176 Atk EVs paired with an Adamant nature lets Rhydon OHKO Landorus-T (minus Intimidate) and 4 HP Garchomp with Ice Punch.
It is worse as a Trick Room attacker though, as Rhyperior's higher damage combined with LO and Solid Rock are preferred for that role. That's why I'm giving it the same rank as opposed to B- rank.
Also Musharna C- --> D rank
It has like nothing over Cress. Only very small niches. Like
-slower Speed and higher SpA, which mean little for a support Pokemon. Even here you compete with Reinculus who has even higher SpA
-Telepathy. The only spread move used at higher level play is EQ, and unlike Levitate, it doesn't protect you from your opponent's EQs
-Imprison, which is never used these days
-other negligble niche moves like After You
It's honestly closer to E rank than C- rank. I myself would never use Musharna as Cress has many more advantages
[edit] Tornadus from B- --> C+
Frail and doesn't hit hard without a Defiant boost.
It fairs poorly against the most common set of the most common Intimidator, Scarf Lando-T, which sucks for a Pokemon that's meant to repel Intimidate. It gets OHKOed by Stone Edge or 2HKOed by Rock Slide, and it cannot OHKO back:
+1 252 Atk Tornadus Acrobatics (110 BP) vs. 4 HP / 0 Def Landorus-T: 270-318 (84.3 - 99.3%) -- guaranteed 2HKO
Bisharp and Milotic do not have these downfalls (Bisharp at least threatens a OHKO on Scarf Lando-T)