I don't think you can exactly say Aero completely outclasses Archeops. Archeops has a massive 140 Attack stat, which, when paired with Flying Gem Acrobatics and STAB Stone Edge, as well as either EQ or Heat Wave, is very deadly in the right conditions. Archeops, aside from being much stronger than Aerodactyl, is capable of running Heat Wave effectively, which means Steelix and Escavalier need to think before coming in, unlike with Aerodactyl. This also allows Archeops to pummel Durant, who is one of the deadlier forces around. Of course, Aerodactyl is faster, and yeah, is generally better, but Archeops still has its place. I think Archeops fits the definition of C to a tree; it has a notable niche, which is to use its STAB's and attack stats (plus its good Speed) to be able to put massive dents in teams, while it has equally as notable flaws in its awful ability, which makes it weak after it loses half of its HP (but that hardly matters because most things will OHKO it anyway, lol), its god awful defensive typing, making it harder to switch in and also making it vulnerable to priority. Therefore, as C-Rank says, Archeops needs significant support to be effective in RU, with the need of a spinner, such as Sandslash or Cryogonal, to keep SR at bay, as well as some bulky team support to be able to take down some priority users. However, with this support, Archeops can be very effective. He also faces competition from Aerodactyl as the fast Rock-type hitter, as you said, which is fine for a C-Rank Pokemon. As for Magneton, I think the reason it's in the C-Rank is because while it sports a very good niche in trapping Steels while also being a potent hard hitter, its rather middling Speed along with three exploitable weaknesses are what let it down. Its typing gives it some very good resistances, but also gives it three nasty weaknesses, to Fighting, Ground, and Fire, which make it easy to kill with threats such as Nidoqueen, Gallade, and Moltres. It is also rather slow, so it may not outspeed much. However, Magneton still has a very good niche, and thus is a C-Rank Pokemon. And on Tangrowth, I believe mid A-Rank is the best place for Tangrowth. He is a very physically bulky Pokemon, and is one of the good physical walls in RU. He can wall some stuff, forms a usable defensive core with Slowking, and has offensive presence, which are all very good perks. However, the arrival of things like Escavalier and Nidoqueen into the tier do give Tang a hard time, so mid is best. I will speak on what should be mid and low B-Rankers later today.