I have a few more which I'm unsure about so I'll think about them and write more on them later, I'll let you know what they are incase anyone has any opinions on them:
Mantyke --> B-tier?
Chinchou --> A-tier?
Porygon --> A-tier?
Croagunk --> A-tier?
Gastly --> C-tier?
Hippopotas --> C-tier?
Abra --> B-tier?
Growlithe --> B-tier?
Shellder --> B-tier?
Helioptile --> C-tier?
So yeah, I've had time to think and read other opinions in this thread and I'm gonna talk about the mons I listed earlier.
Mantyke - not sure because I don't feel I've seen it enough, but as dcae said, it does seem like it'd need more support than other mons in A.
Chinchou - This should
definitely be A. Its scarf set threatens so much, it can keep momentum with volt switch and wall threats such as Murkrow and Magnemite
Porygon -
Probably A, although it is in the lower half of that tier. Paralysis makes this a great support Pokemon as many mons hate paralysis right now. And the ones that are immune to T-Wave are pretty much walled or KOd by Porygon (Chinchou, Gligar, Magnemite) Great answer to Yanma, Missy, Swirlix and more.
Croagunk -
A for sure. Handles Sturdy Smashers, Carvanha, checks Tangela, Swirlix and Fighting types (bar tits), Bunnelby, Pawniard and has access to Icy Wind which discourages Yanma, Gligar and Murkrow from switching into it.
Gastly - not sure because I haven't seen it much, but if someone explained its niche to me, maybe I'd accept it as a B-tier mon. The only noticeable advantage over Misdreavus is a Fairy resistance with STAB Sludge Bomb, but its so frail that it can't survive a +1 Flamethrower after 1 round of LO recoil.
Hippopotas -
Deffo C. Yeah it's annoying for sun - but that's all it is, a slight annoyance. Sun sweepers can just take it out with Giga Drain or a Sleep Powder. As a physical wall it is pretty much outclassed by Gligar, its only advantages being Whirlwind (Torchic check anyone?) and the fact that random HP Ices won't KO it. Its pretty weak offensively and if you do want to use a Sand team, it has to sacrifice its bulk with Smooth Rock, or not give Drilbur enough turns to do damage.
Abra - Yea, I don't have much of an issue with it
remaining C. It seems outclassed by a lot of other fast attackers.
Growlithe - I think this still
could move up to B. It is a great Fighting switch-in, and can burn a lot of shit with Will-o-Wisp. I don't get people using Pawniard as a reason why it isn't good... its not as if you have to switch Growlithe in when Pawniard is out...
Shellder -
Definitely B imo. It can set-up on the most common Pokemon in the tier, Gligar, and then just sweep. It can get past most S-B mons with a bit of support from Knock Off, though Pawniard is still an issue.
Helioptile -
Move up to C. Great Misdreavus check as many only carry Shadow Ball + T-Bolt. Also great in the sun with Solar Power actually making it a pretty good scarfer.
Raseri, Onix, is bulky (on the physical side) but what is it actually walling with it, besides Dwebble, and maybe Bunnelby (who just breaks Sturdy with U-turn)? Most Physical attackers will carry EQ or a fighting move. It can basically take 2 attacks in a clutch situation before dying. It's not really a great answer to Dwebble as well, because it isn't difficult to attack Onix whilst it switches in, breaking its Sturdy and then setting up later on and KOing with EQ.