I personally would say solar once you get enough materials for cables and/or batboxes, MFE's, MSFU's. i made an array of 49 solar panels and it completly filled an MFE worth of EU and then some more EU.
614,376 EU. in one MINECRAFT day. I had set up the array completely at night.
this is vanilla ic2. no addons. unless you count TooManyItems as an addon.