I tend to use the themes I created (as it should be for any author), but here are a bunch that have proved to be really inspirational, fantastic concepts, and a joy to use:

Excalibur II
eXpOSv12
iBDE-V4
Isotanium
le
LSx6.2
Reflection
Simplicity
Tangent Universe
Tenebrae
UniverseMetaLs
VodkaMartini
Widow