Pheresoma is absolutely whack. It pretty much kills everything and if it isn't already outpacing your whole team, after it gets a kill it certainly is thanks to Beast Boost. Aegislash can check it to an extent and so can fairy mons if they're bulky enough and you lack PJab, but it's really futile trying to counter this thing w/out one of those (mainly Aegi).
Aegi's dropping in the first place was controversial, but it's on a majority of teams and it's simply not a very competitive, welcome, or healthy presence in the metagame - it hits hard, it hard stops so many things, it has an incredibly unique defensive presence, etc. - it doesn't really have much of a place in the tier in the long haul, I reckon.
Genesect and Greninja are hard as fuck to counter, too, but they don't seem as urgent or pressing to deal with as either of the above two and they're kinda fun to tinker around with, as well. In addition, Tapu Koko is super fun and incredibly strong for the speed it has - looking for big things from it in the future.
More coming.
-outl