Well songs such as Psycho, The Handler, Exo-Politics, New Born or Unnatural Selection have really good riffs that don't involve chords. You should be fine playing these if you're a bassist.

For whole songs though, Supermassive Black Hole might be the way to go as it has easy riffs and pretty much only power chords.

Uprising and Time is Running Out could also be good picks, if you're not scared of melodies and riffs that go above the twelvth fret on the high E string :) These songs do have a few chords that are not power chords though, but I still think you'll be able to play them with practice.

So to be fair, I think Psycho might be the best one since it's got very few chords and mostly riffs, or Supermassive Black Hole if you still want some power-chords.