Water towers, wind farms, or geothermal banks are going to be your primary options, all of them require a bit of infrastructure.
Geothermal is the most lucrative, however you're probably going to want a pair of Liquid Tesseracts, one of them in the nether and attached to a pump, the other going to the geothermal battery with liquiducts feeding them. At 20 EU/t per generator, it's got the highest EU per block output of any of these options.
Wind Farms are your second most lucrative, but have an engineering challenge in that storms can cause them to break from producing too much EU. Solution is to build them in the desert or other biome without weather. The other problem is that they tend to be built fairly high up for maximum EU production, so piping the EU might be troublesome. You'll likely need at least one transformer to keep from losing it all.
Water Towers are the cheapest option, but has a large 'footprint'. Basically, you have your tin cable running down, and every other block you have four water mills, one on each of the four sides of the cable. Then you just fill everything in with water. Each set of four water mills produces around 1 EU/t, so you'll want to build them in bulk. Build them 30 high (to give yourself a margin for connectivity so you don't lose it all from EU loss) for 15 EU/t per tower.