Some good tanks:
Conkeldurr: With Bulk Up, he becomes pretty hard to hit with physical attacks and can deal big damage with Drain and Ice Punch. Mach Punch can finish off opponents with little HP left.
Tyranitar: Awesome Attack stat, and good Def and Sp.Def stats. Sand stream damage nullifies leftovers and sometimes even focus sash. STAB Crunch and Stone Edge do lots of damage, and you have Earthquake for more coverage. Just watch out for switches into Fighting types.
Bronzong: With 116 Defense and Sp. Defense, Bronzong can become a great tank f the opponent doesn't have any types to counter it. Gyro Ball takes advantage of 33 Speed and 89 Attack base stats.
Gastrodon: High HP, good STABS like Scald and Earth Power, one weakness (although it is a big weakness), access to Recover, and immunity to Water attacks with Storm Drain make for a pretty good tank.
Thundurus: Not the ideal definiton of a tank, but he has good speed, pretty good defenses, and a great Sp. Attack. The fact that he only has two weaknesses and both of them are weak to Fighting AND Steel is a bonus.
I know other people will have more suggestions as well. Hopefully this will help!