I'd go with:
Steelix (Defense: 200)
Cloyster (Defense: 180)
Confagrigus (Defense: 145)
They all have very different weaknesses (In fact, the only overlap is that Steelix and Cloyster are both weak to fighting).
Also, they can have a good variety of move coverage. Steelix would be physical with moves like earthquake, iron head (breeding), stone edge, ice/fire/thunder fang and Crunch. Cloyster could be a mixed attacker. It can learn signal beam, ice beam, and surf for special and physical moves like avalanche and poison jab mong others. It also could set up moves like spikes/toxic spikes. Confagrigus would be a special attacker, plus it has a great special defense. Energy ball, shadow ball, psychic are great, plus it has a huge variety of stat changing moves as well as some status ones.
Go to http://pokemondb.net/pokedex/all and hit "defense" to see a whole list best to worst defense wise to see if something would work better with your team.