I don't usually post in here, but seeing the current Viability Rankings made me wanna make few nominations and agreements on previous ones with a brief explanation, so here it goes:
A- > B+
Looking at it, the only thing Latias has over Latios is Healing Wish, on offensive builds, Scarf Jirachi is better at that almost all the time at that for obvious reasons. As a defogger, it's not that good at that either, seeing how it's forced not to run Roost since HW is basically the reason to use it over Latios.
B+ > A-
Recently, Mega Latias has been getting a really good usage in Tours (OLT and SPL lately), especially with it's BoltBeam Twave Roost set which was found quite good. Combined with it's great defensive stats and a good typing with reliable recovery, it is not easy to take it down even with Pursuit trappers like Tyranitar which fails to ohko and risks coming in on a twave with a good special attack stat with boltbeam coverage that makes it hit most of the tier for at least neutral damage.
B > A-
Amoonguss is simply amazing right now, it barely made it to OU last tier shift and most likely will in the next one. Having a great ability that makes it switch in continuously to threats such as Keldeo and stop some sweepers like Clefable and Mega Altaria. Having great utility moves such as Spore, Stun Spore and Clear Smog with a good defensive typing and STABs makes it a great Pokemon to rely on as a check to a lot at the same time. One of the best niches for Amoonguss at the moment is its viability on sand teams that struggles to switch in to Water types, mainly Keldeo and Rotom-W.
A > A+
I always thought this was A+ for some reason. Tyranitar was always a great Pokemon and always will be. It can serve many rolls atm such as a pursuit trapper for most of the hazard removal Pokemon such as Starmie and the Lati twins or a bulky blanket check to Zardy/Alakazam and Tornadus T with Chople Berry with Stealth Rock and it is considered the main way to get sand up for sand teams, not to forget it's access to a good utility moves like t-wave!
S > A+
I'm on board for this dropping, the meta has adapted in a way that Zard X has a hard time to set up vs most of the teams since Hippo/Lando T/Sand teams in general and even things like Mega Altaria that can switch in and take hits from it. Another thing that hurts it is how bad hazard removal mons are atm. Simply, there are no reliable defoggers/spinners at the moment that can keep Stealth Rock away from the field in the right moment for Zard X to set up. Even it's bulky wisp set not that good anymore and is way less threatening than offensive one.
Also, I heavily support Gastrodon raising for reasons stated in the previous posts.
Finally, not really a nomination, but I can see Keldeo being S in the near future again and not going to state reasons right now, will wait a bit on that.