I would like to nominate Jelicent for A Rank
It HARD WALLS any Keldeo set, and the Sub CM set can't do anything to it, while Jellicent can Taunt it and render it unable to even set up to +6.
Note how a B+ Ranker can HARD WALL an S Rank sweeper. The only other pokemon that can HARD WALL an S Rank sweeper is Landorus-Therian vs. Terrakion. And Mr. L. Therian is in A rank.
Jellicent can also run a great special defensive set which beats Sheer Force Landorus (EP 3HKOs, Jelly can 2HKO back with Scald/Surf or has a 37.5% chance to OHKO with Ice Beam), a Choice Specs set with that surprise factor, or the standard Utility Counter set (which is my favorite Jellicent set).
Having reliable recovery is a must in the fast paced metagame of BW2, and this puts it apart from its water type bretheren like Suicune and Gyarados.
Jellicent can even stand to Thundurus-T's Thunderbolt, the strongest electric type special attack in the game. Against NP/Expert Belt/Scar variants, Special Defensive Jellicent do not face an OHKO, while it can deal decent damage with Ice Beam, weakening it enough to be in OHKO range of most priority moves bar Breloom's Mach Punch.
Jellicent's typing allows it to block Rapid Spin, crucial for certain sweepers like Thundurus-Therian, Volcarona and Salamence.
Jellicent also beats Starmie and Donphan, common spinners. Starmie's Thunderbolt fails to 2HKO Special Defensive Jelly, while it can 2HKO it back with Shadow Ball. Donphan offensive or defensive, fears the 2HKO from Scald/Surf and its main purpose, to spin, has been destroyed. While Donphan's Seed Bomb is stronger than Starmie's Tbolt, Utility Counter Jelly isn't even 2HKOed by that either.
And Donphan's Seed Bomb got 7.970% usage in the March OU Usage stats, so Special Defensive Jelly basically walls Donphan. Thats pretty good since every successful Sun Team has Ninetales, Venusaur, and Donphan.