Tywin Lannister and Theon Stark.

Who exactly outranks who as far as ruthlessness is concerned?

1.) Theon Stark: After defeating the Andals in battle, sailed to Andalos, burned dozens of villages to the ground, as a well as a Sept, then mounted the heads of those he slew on pikes and displayed them all along his coast.

2.) Tywin Lannister: After defeating them in battle, brought down the Tarbeck's walls and burnt their castle to the ground, and then flooded Castamere drowning everyone inside.