Solar is nice, as a backup. They cost a lot to craft for a small output, but that's because they run for free and are truly a "set and forget" energy source.
When I'm starting up, I usually build solars. Even if they only work for about half a day, they still are passive EU which is nice.
Though without CompactSolars they tend to cause massive amounts of lag from eNet updates from crossed wires(which is easy enough to fix, just paint them different colors, assuming you aren't using tin[thus you'd pretty much need glass fiber cables since no other cable has low enough EU loss to allow for big solar flowers]), but the same can happen WITH CompactSolars, it's just less likely as you reduce the amount of blocks needed to get the same amount of power, thus reducing the eNet load. If one builds a solar flower with CompactSolars, they quite clearly need to be shot, though.
But with Redpower and Nuclear Control, one can build some amazingly good reactors. Add Thermal Expansion and you can make a compact yet efficient Ice CASUC design, even without compressors(though it is a good idea to use compressors, makes it faster to get the ice, but it isn't really needed). Add ComputerCraft and the CCSensors addon and you can build a computer which can controls the whole reactor, no switches or buttons.(though one might be able to do that with RedPower Computers) One could even use the Force Field Generator mod(the old version, anyway) to automate the cooling and heating system and allow for one to prevent massive explosions from reactors. The new version doesn't have that feature, but at least someone is maintaining it.
Mix those together, and you'll have a super powerful reactor which you can turn on with the press of a button, and generate enough power to fill as many MSFUs as you want/have, and it'll automatically shut down when it has filled them. And it's still cheaper then enough solars to generate enough power to match the average EU.(keep in mind solars work for about 50% of the day, so even if you get 2000/EU a tick from your solar plant, you really need to bring in 4000EU/tick to compete since solars don't work at night, thus driving the cost up)
As for geothermal, well shit, it's basically free anyway(there's more then enough lava in the world, even without draining the nether), and it works great.
If only geothermal power was realistic in how it works in IC2, one could say "It's worth the risk" and not have to bite your cheek. Mind you, if IC2 was realistic, that would kind of beat the point of using it.