Blue Jay
The notorious Good Wife
A few things to correct here.This post might seem controversial, but I think that Xerneas should be dropped to A+ rank. Here's why:
1. Xerneas absolutely needs to use a power herb in order to use geomancy, but without life orb i feel like it doesn't hit hard enough...
2. Xerneas is easily revenge killed by arceus-normal and CB Scizor, which were very viable in gen 5 too. It also can't really get past mega gengar.
3. Xerneas suffers from 4 moveslot syndrome. Without thunder(bolt), it has no means of doing anything to Ho-oh (+2 geomancy doesn't 2hko), while brave bird is an easy OHKO with just a bit of prior damage. additionally, Ho-oh can easily Whirlwind it out. Lugia also completely walls non-Thunder(bolt) variants if sr isn't down, and guess what, Lugia can easily roost. However, xerneas really needs moonblast, focus blast and flash cannon to beat the most common threats
4. Xerneas can't really boost to +3 SpA/+3SpD/+3Spe, which might be crucial at times (i know it's situational, though)
Gengar is the one that needs to watch out for Geomancy Xerneas, if Xerneas sets up safely a Thunder or Psyshock will blast Gengar to bits. The only way Gengar can handle Geomancy Xerneas is if it lacks moves that OHKO Gengar after a boost (and even then it will have to sacrifice itself to eliminate Xerneas, as Sludge Wave does not come close to a OHKO against +2 Xerneas), or if you somehow manage to prevent Xerneas from setting up at all, which is rather challenging to do.
Xerneas also does not need Flash Cannon for anything. It has awful coverage, middling Base Power and is actually weaker than Moonblast against opposing Xerneas due to both Pokemon's Fairy Aura boosting the power of Moonblast. Thunder(bolt), Focus Blast, Hidden Power Fire and Psyshock are the sensible options, but most targets for Psyshock are awful, rare Pokemon and Focus Blast and Hidden Power Fire overlap a good deal, so the 4MSS is not really that bad. Thunderbolt also accomplishes nothing against Lugia; in fact, because of how powerful Moonblast is, Ho-Oh and Heatran (which would be hit much harder by Focus Blast instead) are about the only Ubers viable Pokemon against which Thunderbolt is more powerful than Moonblast.
Furthermore, please acknowledge all of Xerneas' viable sets. It is rather versatile.