Ok, so I'm going to tackle the 3 Pokemon that have been nominated for S Rank. I'm not even sure, so maybe talking about them will help me find out where I think they should go.
S Rank: Reserved for Pokemon that are amazing in the OU metagame. These Pokemon are usually able to perform a variety of roles effectively, or can just do one extremely well. Their use has low risk involved and high reward exerted. Pokemon in this rank have very few flaws that are patched up by numerous positive traits.
First, Latios. If Latios were to go to S Rank, it would be because there are a fair few roles it can preform effectively. There's the hard hitting Specs Trick set that serves as a useful wallbreaker, a Scarfed set that can be a nice revenge killer/cleaner that checks a fair few Pokemon and an offensive deffoging set. All of these roles, I must say, are pulled off very well by Latios. It's got a nice movepool to support all of these sets, checks/wallbreaks a fair amount of the metagame (depending on the set) and puts on A LOT of offensive pressure as a defogger. However, the huge list of Latios switch-ins makes this one interesting. Things like Tyranitar, Chansey, Clefable, ext have no trouble switching in on this thing (the latter 2 have to watch out for Trick sets), and those Pokemon are VERY common in the metagame right now because they are good. It certainly limits Latios' usefulness, seeing as 1 of those is a Pursuit Trapper. It's also checked by a huge amount of Pokemon in this metagame (bar the scarf set but that's it's worse set) such as Greninja, Scarf Garchomp and Scarf Landourus-T. There's also the fact that Latios has too many grinding flaws to be S Rank. It hasn't got very spammable attacks for an offensive Pokemon, it's a defogger for who the most part loses to every pursuit trapper bar maybe Scizor if it has HP Fire, ext. Overall, I personally think Latios would be at S Rank standard, but all the things that switch in to it and all the things that check it are very common in this metagame, so it can't be S Rank. I didn't have to go into the whole risk vs reward thing (which is in Latios' favour) but the fact of it being dealt with so easily in the metagame limits it's usefulness and prevents it from being S Rank.
Now onto his sister, Latias. Unlike her brother, would be S Rank for performing one role extremely well, and that role is defogging. Now I hope you all agree with me, but I personally think that Latias herself is the best defogger in the entire game, and that's because of her access to Healing Wish. This is a truly fantastic supporting move for offensive teams, and is one of the reasons Latias is so easy to slap on offence, along with her still bringing offensive pressure to the table. She's fantastic... but suffers from the same thing as Latios, only worse. So many switch ins, so much that checks it, ext. Latias also finds it harder to break through common walls unlike her brother. She DOES have the edge vs pursuit trappers though as she always has Healing Wish, but it's still an insta death, even more so than it is with Latios. Overall, while Latias is the best defogger in the game, I feel like it has too many grinding falls AND doesn't have low risk high reward. There is risk, seeing as something like Clefable could just switch in when you want to defog and set up CMs, and Clefable isn't the only Pokemon to do this. Latias and her brother stay in A+.
Onto Mega Pinsir. Unlike the last to, Mega Pinsir for S is really something I could see happening. It's got 1 role that it preforms almost perfectly, with VERY few flaws, and that is wallbreaking. I doubt anyone could argue otherwise, seeing as it's counters (Skarmory and Latios) are falling in viability or a destroyed by his commonly seen partners. Mega Pinsir has 2 sets, the Sub set and the SD set, both of which wallbreak very well, about as well if not better than the Mega Wallbreakers. Together, they make Mega Pinsir's wallbreaking standards S Rank. As for risk vs reward, when using Pinsir, due to the lack of switch ins and limited checks due to Quick Attack/Substitute, I feel like there is little risk using this thing and very high reward exerted. In other words, I feel as if Mega Pinsir passes the criteria with flying colours. Mega Pinsir for S Rank.
Later on in the day I'll be talking about why a few of the S Rank Pokemon could move down.