I think Feraligatr basically teeters on the border between B/C rank.
On one hand, it resides among the best priority sweepers; Aqua Jet generally has superior neutral coverage than Mach Punch or Bullet Punch, and all of the viable ExtremeSpeed users don't get STAB on the move. Given Feraligatr's good but not outstanding bulk, most strong attacks in OU will put it into Torrent range from full health. Factoring in Torrent and rain, its Aqua Jet is actually the strongest priority move in OU, possessing a whopping 135 BP with STAB, which can be boosted further by holding a Splash Plate.
On the other hand, it requires extensive team support to work since there are many things that can check it. Most teams have at least 2/3 Water resists, almost of all of which can comfortably take a +2 Torrent-boosted Aqua Jet in the rain at full health. You have to work hard to weaken these as a smart player will almost certainly try to keep his main Feraligatr check healthy at all costs. Stealth Rock support is absolutely mandatory for this set to work, though Spikes are recommended too - at least 2 layers is ideal. You also need to remove faster priority users - Breloom, Lucario and Dragonite can all easily revenge-kill Feraligatr. Also, depending on whether you opted for Crunch or Superpower, different things will hard wall you so you need to plan the rest of your team carefully so that these things (i.e. Ferrothorn, Jellicent etc) can be dealt with. You could run both, but Waterfall is very useful for breaking through slower, bulkier Pokémon as it hits like an absolute truck with rain and Torrent taken into account (it's as powerful as Victini's STAB V-Create). It can even be used as the main sweeping move with some Thunder Wave and/or Tailwind support, as at +2 it OHKOs everything bar the bulkiest Water resists (though not to replace Aqua Jet's moveslot, mind you).
Based on my own experience, although Feraligatr can sometimes be difficult to use, and requires you to build a team around it, I think it can just about make that push into B-rank as it does have a definite niche that has been recognised by some of the top-level players. I don't mind if it ends up in C-rank though.