Really Kingdra can do so much more than just counter rain teams guys...The LO rain dance set also makes him an amazing sun counter as well, neudering a sun team and than sweeping with draco meteors/pulses or ice beams. It also makes him not just dead weight against a balanced team as after a rain dance he cant be revenged by a scarfer and can blast off rain boosted stab water moves, which backed by a modest nature and lo can do about 40% to ferrothorn, which is pretty impressive. And remember, unlike other dragons kingdra has stab water moves. So to say he's completely outclassed by other dragons except against rain teams I really disagree with. As for its ranking I think B fits it perfectly well, as its semi-low offensive stats and fratility really hold him back from being a top threat.
tl;dr: Keep Kingdra in B, but he's more than just a rain counter
PS name- uberscape