..okay. Since it's apparent that everyone will want their "favorites" moved up until nothing exists in any rank below C, I'm going to try to introduce some objectivity into these proceedings.
S Rank: Reserved for Pokemon who can sweep or wall significant portions of the metagame with little support, and Pokemon who can support other Pokemon with very little opportunity cost ("free turns"). Also the home of Pokemon who can easily perform multiple roles effectively, increasing their versatility and unpredictability. If the Pokemon in this rank have any flaws, those flaws are thoroughly mitigated by their substantial strengths. If there are suspects, they will come from this rank.
A Rank: Reserved for Pokemon who can sweep or wall significant portions of the metagame, but require some support or have some flaws that prevents them from doing this consistently. Supporting Pokemon in this rank may give opponents free turns or cannot create free turns easily themselves, but can still do their job most of the time.
B Rank: Reserved for Pokemon who cannot sweep through or wall significant portions of the metagame, but can properly fulfill a given offensive/defensive niche. Support Pokemon in this category have flaws that prevent them from doing their job or are setup bait for dangerous sweepers. Pokemon who are partially outclassed by a Pokemon in A or S Rank, but are otherwise very dangerous, may also fall into this category.
C Rank: Reserved for Pokemon who can be effective given the right support, but either have crippling flaws that prevent them from consistently executing their strategy or are completely outclassed by a Pokemon in the above ranks.
D Rank: Reserved for Pokemon who are simply not very effective in the current metagame.
Some examples of the above. These are just my (educated) opinion, but they should give you an example of what I interpret the ranks to mean:
S Rank: Milotic (Defensive), Registeel (Defensive), Uxie (Support), Venusaur (Versatile)
A Rank: Moltres (Offensive), Donphan (Defensive), Clefable (Support), Rotom (Versatile)
B Rank: Magmortar (Offensive), Slowbro/king (Defensive), Dugtrio (Support)
C Rank: Charizard (Offensive), Tangrowth (Defensive), Venomoth (Support)
D Rank: Nidoking (Offensive), Bastiodon (Defensive), Wigglytuff (Support)
Thoughts? I'm open to debate on most of this, but I think it's vital for us to establish some objective criteria if we want this to be a serious thread.