Abomasnow for A Rank. Someone mentioned earlier that Ninetales was the worst weather starter, so shouldn't Abomasonow be in the same tier? I freely admit that Drought>Snow Warning, but Abomasnow is a really underrated Pokemon that gives you a solid weapon against all weathers (even Sun). The yeti may have an abominable seven weaknesses, but the resit to Water and Grass STAB give it the upper hand vs. rain teams. The Ground resist is handy in general, letting it take Earthquakes when switching in, and it threatens most users of the move with its STABs. It does well vs. Sand also-- Tyranitar fears Wood Hammer, and Hippowdon doesn't want to switch into a Blizzard (despite the 70% accuracy because of Sandstream). Abomasnow also has STAB Ice Shard, giving you a check to dragons, genies, and Chlorophyll 'mons. And Hail is a nifty weather to have around, as few Pokemon in OU are immune to the damage. Abomasnow also can run a variety of sets: Scarf, Band, Leech Seed, and Expert Belt are all viable.
And a completely unrelated Pokemon, Cradily for D Rank. It has a small niche as a special wall and anti-Rain Pokemon for Sand teams, because of its nice 86/97/107 Defenses (and its SpD is further boosted because of its Rock typing) and Storm Drain. Its Grass/Rock typing is unfortunately defensively, with weaknesses to Ice, Fighting, Steel and Bug, and with resistances only to Electric and Normal, but in Sandstorm its special bulk lets it take even non-STAB super-effective attacks with ease and Recover off the damage. Its Grass typing gives it a STAB Giga Drain, letting it threaten rain teams, abuse Strom Drain boosts, and heal simultaneously, and it can use its secondary Rock STAB for coverage vs. Flying-, Bug-, Ice- and Fire- types (and if it gets the Ancientpower boosts it becomes almost unstoppable). It has a small niche, but it is very good at filling it.
And a completely unrelated Pokemon, Cradily for D Rank. It has a small niche as a special wall and anti-Rain Pokemon for Sand teams, because of its nice 86/97/107 Defenses (and its SpD is further boosted because of its Rock typing) and Storm Drain. Its Grass/Rock typing is unfortunately defensively, with weaknesses to Ice, Fighting, Steel and Bug, and with resistances only to Electric and Normal, but in Sandstorm its special bulk lets it take even non-STAB super-effective attacks with ease and Recover off the damage. Its Grass typing gives it a STAB Giga Drain, letting it threaten rain teams, abuse Strom Drain boosts, and heal simultaneously, and it can use its secondary Rock STAB for coverage vs. Flying-, Bug-, Ice- and Fire- types (and if it gets the Ancientpower boosts it becomes almost unstoppable). It has a small niche, but it is very good at filling it.