Right now on my SMP server one would find a very good mix of power sources. This server uses IC2, BC, RP, Forestry, Railcraft, and Additional pipes.
Lava is my primary fuel source for both my BC and Forestry engines. Biofuel (Forestry) going straight to to bio-engines to convert it to EU, BC Fuel used to power any quarries or fillers using combustion engines and teleport pipes.
Geo= My go to for new starts, but also supplemental power right up until Nuclear. I generally strip must maps clean of lava around my main bases for many many chunks.
Wind= If my base is near 100 I'll build a respectable farm, I like the high constant yield and the chance of these to "burning out"
Water= used in many situations to even include main power for smaller satellite bases.
Solar= vary rarely used, since I only sleep once to set bind and generally play on SMP, they are just too useless for my taste.
BioFuel (IC fuel cans) = used in most most mid-tier bases as a power boost, depending on the output of my farms.
Generators = never used at the start of a base, only used in conjunction with excess wood (as charcoal) from Forestry.
Biofuel (Forestry) = my go to supplemental power through out the life of my main bases.
Cross over mods= I find ALL of them besides Forestry to be way over powered. I won't even install them on my SMP server.
Even back in IC1 I would use a mix of geo, solar, and wind. I never like to depend on any one source.