Someone with Toxic and/or Toxic Spikes can be very useful to whittle down tough customers.
Dragalge is fairly bulky and packs a punch, and so does Drapion, but you're probably better off with a grass/poison to avoid getting swept by ground types (Venosaur, perhaps Amoonguss).
Actually, it's a good idea to prepare someone able to Mega evolve since sooner or later we'll get the Mega Ring (Edit: oops, forgot about Mega-Medicham).
Ultimately, I find useful to have at least a Pokemon able to set up and sweep (anything with Shell Smash, Dragon Dance, Quiver Dance, Shift Gear and the like).
As for your candidates, Jellicent is pretty good, and there are crests for both Cofagrigus and Dusknoir. Clefable with Cosmic power and Stored Power can be a sweeper in its own right, as long as you manage to avoid critical hits.