For the record, I've changed my mind on Nidoking and think it should be A, mostly because Megahorn is awesome and makes it not outclassed by Nidoqueen.
With regards to Mega Aerodactyl, I think it should be A- Rank. 4MSS is ass, and stops it from being as effective as it wants to, when it wants to run Stone Edge / Aerial Ace / Aqua Tail / Earthquake / Crunch / Taunt / Roost / Defog / Stealth Rock. When looking at the A rank Pokemon, it really doesn't stack up. Nidoking is a wallbreaker that doesn't really have trouble with moveslots, is stronger, and is more threatening to defensive cores. Forretress and Umbreon are better supporting Pokemon that can do their job without trouble and do more for a team. Suicune, Darmanitan, and Honchkrow are metagame defining threats that you need to have a strong answer for, or you will be swept. In contrast, most teams can handle Mega Aerodactyl, either with a scarfer, or just with a pretty physically bulky Pokemon. While it has the ability to threaten many walls in theory, in practice it has to pick, and while that's also nice for it, it limits its options and how threatening it is. Really, the main difference is that with an A rank Pokemon, you need to have a specific strong answer to it. Mega Aerodactyl has too many answers for that to be the case. It's easier to counter, and it fits better in A-. It still is a big threat and offers great support, it's just not as big of a threat.
With regards to Mega Aerodactyl, I think it should be A- Rank. 4MSS is ass, and stops it from being as effective as it wants to, when it wants to run Stone Edge / Aerial Ace / Aqua Tail / Earthquake / Crunch / Taunt / Roost / Defog / Stealth Rock. When looking at the A rank Pokemon, it really doesn't stack up. Nidoking is a wallbreaker that doesn't really have trouble with moveslots, is stronger, and is more threatening to defensive cores. Forretress and Umbreon are better supporting Pokemon that can do their job without trouble and do more for a team. Suicune, Darmanitan, and Honchkrow are metagame defining threats that you need to have a strong answer for, or you will be swept. In contrast, most teams can handle Mega Aerodactyl, either with a scarfer, or just with a pretty physically bulky Pokemon. While it has the ability to threaten many walls in theory, in practice it has to pick, and while that's also nice for it, it limits its options and how threatening it is. Really, the main difference is that with an A rank Pokemon, you need to have a specific strong answer to it. Mega Aerodactyl has too many answers for that to be the case. It's easier to counter, and it fits better in A-. It still is a big threat and offers great support, it's just not as big of a threat.