I'm going to support the notion to move Heracross down from S-Tier to A-tier, and support the moving of Cofagrigus from A-tier to S-Tier. I've enclosed my arguments in spoilers to help organize this post. I do not believe that Raikou should be dropped, since it is the pinnacle of Special Attacking in the Underused tier, possessing superb coverage and being more than capable of dismantling the tier with next-to-no support.
In a tier already heavily dominated by Fighting-types, Heracross stands among a select few who can really lay down the hurt in the Underused tier. However, it is no longer the best, as it's been usurped by Mienshao's wider movepool, greater Speed, and ability to bypass many checks that Heracross would otherwise fold against (namely Gligar). I've based my argument on 2 points, those being how the tier has adapted to Heracross, and how effective Heracross really is in the modern Underused metagame.
Ever since the release of Moxie Heracross, it's been something to always keep in mind when teambuilding and battling. However, in the 8 or so months its been around (yeah, that long), the tier has shifted to almost guarantee Heracross will be of no use until the final stages of a match. Gligar's usage spiked, as it is a full counter to nearly everything Heracross would dare throw at it. Cofagrigus rose from the peaks of RU to combat Heracross, and has inevitably won that battle. Qwilfish, another RU Pokemon, became another solid check to Heracross, packing Intimidate and only fearing Earthquake. Crobat became notoriously good for being able to eat anything but a Stone Edge and bust Heracross' horn with Brave Bird. Zapdos followed suit with Crobat, as it was also capable of nuking Heracross with Heat Wave or Hidden Power Flying. Mienshao was given Aerial Ace in order to warrant the revenge kill of Heracross. Nidoqueen's usage was given a slight revival, as it was adept at switching in on Heracross' STAB. In short, what I'm trying to get at is that Heracross isn't as safe as it used to be in such an ever-changing tier. The tier violently searched for ways to find a way to neuter the bug, and well, it came out in spades. everyone and their mother was and still is using 2 or 3 checks for Heracross, simply because it backed such a large punch. Its base 85 Speed quickly became a benchmark for other Pokemon to keep pace with, and nothing's really looked back since. Sableye, Cofagrigus, and Mew all became common checks to Heracross, as their Will-O-Wisp was more than capable of keeping one safe from that ever-looming threat in Heracross. This is why I'm glad we didn't quickban Heracross, as it was one of the last things introduced to the tier to inspire some creative teambuilding, using RU Pokemon to handle the metagame's former biggest threat (who's title probably belongs to Victini now).
Fast forward 8 months since Heracross really took the tier by the balls, and you'll see it just sorta bumming around, being used namely for revenge killing and cleaning house in the late game. The vast influx of checks and how predictable Heracross became really took a sizeable chunk out of how effective Heracross was. Everything listed in the current S-Tier is pretty much capable of beating Heracross 1-on-1, the exception being Snorlax, and many Pokemon in the A- and B-tiers follow suit. Even its other sets, such as Swords Dance and Choice Band, are a huge liability, as they open the door for a new wave of checks, such as Azelf, Arcanine, and other sets commonly outsped by the Scarf set, to come in and quickly eliminate Heracross. Heracross has never been well-known for its bulk either, as it's 2HKOed by many a threat in the tier. Many formerly uncommon sets have seen an increase in usage, such as Rain Dance Kingdra and OTR Cofagrigus, allowing Heracross to be outsped and eliminated with ease. With very few opportunities to switch in, and many Pokemon being paired or carrying coverage moves that will beat obvious Heracross switches, Heracross shouldn't be seen as a top tier threat anymore. It's a cured illness.
I also want to note that teams that get 6-0'd by Heracross are not a showcase of how good Heracross is, but rather that it's just an exceptionally poor teambuild that couldn't be bothered to check one of the bigger threats in the tier.
Ever since Research Week Mk. 1 #5, it was clear that Cofagrigus was going to be Underused. Its ability to check every physical attacker in the tier (bar Darmanitan and Victini, since there's only like 6 Pokemon in existence that can actually do it) and massive offensive presence in the form of an Offensive Trick Room set would literally raise the bar on how to be a Ghost-type in UU. Regarding Cofagrigus, there isn't much of an argument that needs to be had to prove that it's S-Tier material - its the Special counterpart of Snorlax, which is reason enough on its own. With access to moves such as Haze and Will-O-Wisp, it's the true defensive hazer, as it's even capable of stomaching many Special Attacks while being capable of doing something in return to an opponent. Its ability Mummy cripples a wide number of threats, especially Azumaril, Sharpedo, Mienshao, and Heracross. Cofagrigus was capable of jumping into a higher tier and becoming a number one defensive Pokemon in a matter of less than half a year, which is an extraordinary feat by my books. It's the new Heracross, having grabbed the physical attackers of UU by the balls, dictating when a Fighting-type can and cannot fire of powerful STAB attacks, and effectively solving the Ambipom issue. By definition, Cofagrigus is more than deserving of the title S-Rank, as it truly is a Pokemon that needs next to no support if it wants to sweep or wall an opponent.