Try to cover what I feel experienced enough with:
Delphox: I am pretty torn on this one. On the one hand, Delphox has a difficult time with some of the top Pokemon in the tier (Zoroark, Shark, Jolteon, Slowking, Druddigon to an extent, etc.) but on the other, Delphox is one of few offensive Pokemon that can double as a wallbreaker and a sweeper with one set (LO Calm Mind) If you don't have AV Slowking or AV Druddigon, the Specs set is just obliterating something. Psyshock even makes AV Druddigon's life a bit tricky. While it probably isn't quite as good as it was when it first moved to S rank, I still feel Delphox is a good enough offensive Pokemon to warrant a spot in S rank. I know it has a hard time with Zoroark and AV Slowking, but without AV Slowking, you don't have a fullproof switch-in to its best set, which is LO Calm Mind (it typically runs Fire Blast, Psyshock, and Grass Knot) and if the opponent has a Zoroark (or some other faster physical attacker) just play LO Calm Mind as a wallbreaker.
Amoonguss: Amoonguss is definitely a reliable Pokemon right now. It's capable of forming great defensive and balance cores quite easily thanks to its typing. It's a great mixed wall capable of working well with the likes of Slowking, Moltres, Rhyperior, and more thanks to its typing, bulk, and ability in Regenerator. Spore is fantastic support and Clear Smog is really nice for boosting sweepers that don't use Substitute. It suffers from being weak to a fair few top threats in Delphox, Yanmega, and Moltres (among others) but it can also serve as a reliable check to others, like Rhyperior, Sharpedo, and Virizion. Solid A rank Pokemon that's rather easy to form balanced and defensive cores with imo
Moltres: I consider myself one of the more experienced Moltres players around as my oldest team features one. Anyway, Moltres is a fantastic Pokemon. With a fantastic Special Attack, great dual STAB combination, underrated defensive typing, usable defenses, and respectable Speed, Moltres seemingly has everything it needs to be S rank. The problem? A few actually. First of all would be the Stealth Rock weakness. Now, this may not seem like a huge issue considering that Hitmonlee and Kabutops have good synergy with Tres and have Rapid Spin, but they also have an unfavorable matchup vs. Doublade, the most common spinblocker in the tier. Combine this with the fact that Moltres doesn't have good synergy with most Defog users, outside of Shiftry, and it can be a bit of a challenge to control hazards when using Moltres. Next would be accuracy. My god accuracy sucks. Try going a whole match, using Hurricane 6 times, AND MISSING 5 OF THEM. Yeah, accuracy issues are real and will generally betray you at the worst possible time. The last is AV Slowking. So long as this guy is around, Moltres isn't breaking walls or sweeping any time soon. Specs is just a poor choice on a 4x Rocks weak Pokemon when LO gives you the same notable KOes outside of King (and you're relying on Hurricane hitting twice in a row...) Moltres is fantastic, but it has a few flaws that hold it back from S rank imo.
Jolteon: I used this on a recent team of mine that I used to test something else out, and I was a bit surprised at how well it held its own. Jolteon has great utility on offensive teams as it's the only viable offensive Wish passer in the tier. If Wish passing isn't your cup of tea, then perhaps fast Volt Switches are. Jolteon outspeeds the entire unboosted metagame sans Accelgor (and random aids crap like Electrode and Ninjask) while having just enough coverage to get by, making it a very reliable late-game cleaner. Jolteon happens to work well with two of the best special attacking wallbreakers, Moltres and Clawitzer, and is able to either support them with Wish (especially useful for Clawitzer) or simply take advantage of their wallbreaking prowess by cleaning up what they leave behind. Jolteon's great natural Speed, solid power, offensive utility in the form of Wish and Volt Switch, and access to just enough coverage to clean late-game gives it enough for A rank imo.