Reasons that Zangooser stated above ^
I will give a brief explanation for each:
S+
: Arguably the best mon in AG atm, due to the sheer versatility it can bring to the battlefield. Lum, Life Orb, Silk Scarf, Leftovers, you never know what set it is exactly running especially when their is more than one of them. Able to sweep teams with Ekiller, while also being able to stallbreak with it's
taunt+will-o-wisp set. Extremely splashable, making it that there is no reason not to use it.
S
: Use to be considered the best mon in AG, but now has fallen due to Arceus slightly outclassing it in some sets. Still an amazing wallbreaker with it's lum+SD set, dealing impressive amount of damage practically 2HKOing the whole tier with a life orb. It is mostly prepared for now however.
: Its Geo+Herb set is truly amazing, being one of the best set up sweepers in the tier. It is very difficult to stop it once it sets up a Geo, with a decent movepool that can help deal with some checks and counters who resist Moonblast. But every decent team has an answer for it one way or another however.
: Very good utility mon, possessing a great typing with an ability that compliments it. Is used to check some of the meta's most prominent threats in Xerneas, Ekiller, Klefki and Mega Diancie. Also possesives other greats sets in Double Dance and it's lure set. Another splashable mon, which requires scouting to figure out it's exact set.
S-
: Being able to spread sleep like crazy, due to it having a high base 130 spd stat. It's NP set can rip through unprepared teams not carrying a magic bounce user. Is weak to strong priority attacks however, as it possess poor bulk.
: Keyspam is just so good in this meta, that every serious AG MUST have an answer for it to be effective. T-Wave, Foul Play, Swagger, and Substitute or other move to deal in it's checks and counters. Very annoying mon, which is pretty prepared for however.
: Being able to check the omnipresent keyspam and Darkrai's in the tier. While also possessing a decent offensive typing, with it's RP or CM sets breaking through stall and balance alike. Although some consider it being outclassed by Ray as a mega.
: One of the bulkiest mons in the meta, being able to face the most prominent threats in Ekiller, P-Don, Xern, and Mega Ray and phase them out. It can also run an effective CM set as a surprise factor to sweep unprepared teams.
Hope this helps :)