What pokemon should i teach TM Earthquake in Platinum?

Depends. Infernape is a good option for it but it's super effective against rock, steel and electricity. Infernape pretty much takes care rock and steel with STAB super effective moves which leaves electricity, but it's ok to have a variety of moves. Gastrodon has more of a Special Attack stat so Earth Power suits it best. Tangrowth can have it to take care of Fire and Steel types, though most of the time the fire types can outspeed it. Bronzong and Drapion are the best choices though, Bronzong can probably take a hit from a fire type and pay back with Earhtquake, and Drapion is just a physical sweeper and can use it effectively.