What was the first power cable that you noticed a difference in the sound?

Nordost Blue Heaven.  When I connected them in, much blacker background.  That showed me power cords make a difference. As my system has changed so have the cables.  I am now running full loom of Kubala Sosna Emotion.  Best move I ever made. 

Kubala Sosna Elation! I think they are $2500.

Shunyata Black Mamba on sources and 2 BMI Whale Elite on amp which requires 2 power cords. Much quieter back drop the music was presented in. I still have them. I use Wireworld Silver Electra 7's on my digital front end now. 

ZU Event Power Cables. Not an improvement of sound character so to speak (low, mids, highs, etc). The improvement came with much lower noise. The music seemed to come out of more darkness. Very subtle but worth it. 

Audio Envy Ocean Elite Expanded With OCC ground upgrade.  Music just took several very quiet steps out into the room.  Extremely nice cables at such reasonable prices.  OCC wire , quality insulation, gold plated solid copper tight fitting connectors and very light and flexible cable.