Depends on what your team needs:
Tyranitar and Hippo are both great to set up rocks.
However, that being said, they also lead different roles...
a well-known set this guy uses is a Dragon Dance set. after 2 DD's, he can basically destroy anything on the field.
this dude works works great when stalling. A nice Toxic + Slack Off + Protect can mess the foe up real bad. Also, he can work well with Stealth Rocks + Whirlwind.
Both are great, just depends on what your team needs. However, Hippo only has 3 weaknesses, while Tyranitar has 6.