I must say, I think Mercy is one of their best ever pop songs. It has really grown on me, extremely catchy, more heavy than starlight for all the right reasons and combines a lot of great instrumentation. Fantastic bass line, great guitar in the chorus and a fantastic melody, all backed by the lovely synth appregios.
Compared to revolt it's a no contest. No matter how often I listen to revolt I simply cannot get past the mountainous wall of cheese facing me. The bad kind of catchy.
I actually also love Dead Inside. First half isn't amazing, but those vocals from the bridge onwards is some of Bellamy's best.