Rampardos or Shuckle?
Rampardos- Well, first off, Rampardos' Attack is awesome with a base Attack of 165. However, the rest of its stats, ehhhh, not so much. If Rampardos is lucky enough to survive an Attack, it could easily OHKO a Pokémon.
Shuckle- Uh, hello? 230 in Defense AND Sp. Defense? Easily the most Defensive and Specially Defensive Pokémon. However, it's Attack, Sp. Attack, and Speed are terrible. Could this be used to it's advantage...?
Now, on to Movesets!
Rampardos- It's just average. It's best moves are probably Head Smash and Zen Headbutt. It would probably get it's best moves from TM's (my preferred moves would be Smack Down and Earthquake).
Shuckle- It actually isn't that bad! It's access to Power Trick, Guard Split, and Power Split could make it unstoppable, if it used Power Split and then Power Trick or Power Trick and then Guard Split (It would have terrible Defense! Give the opponent that horrible Defense, and Shuckle can OHKO it. Also, having terrible Speed, using Trick Room before using it would DEFINETLY make it unstoppable.
All together, I would definetly go with Shuckle.