Personally, I'm a HUGE fan of Mekanism, and I consider it a strict improvement over IC2.
You don't have complicated and convoluted mechanics. You have your energy, it flows, it runs things. No needing to worry about things blowing up.
It's great at the early game, the Heat Generator is a really amazing machine. It's cheap at the price of one redstone and one osmium ingot and some cobble and furnaces, and you can either use it as a bog-standard engine that runs off of burnables, or you can surround it with lava for around 25 J/t passive energy.
Then you get the Metallurgic Infuser, which is used to make steel and a few other things you'll need to set up your infrastructure.
From there, you can start making your Enrichment Chamber for ore doubling and your Powered Furnace.
Once you do that, you can get your Crusher going, and probably upgrade your power to Hydrogen Generators, using the power from the Heat Generators to power the Electrolytic Separators for the hydrogen generation. While you're at it, you can build another Separator for your Purification Chamber, and now you've got triple ore production.
You've also got, for powered tools, the Molecular Whazzitz that pretty much goes through everything like a hot knife through butter. You've also got fairly early access to a jet pack (although the fuel supply is rather limited), and fall damage negation boots.
At this point, the only thing it is truly missing is documentation, which I might end up doing myself just to save everyone the hassle.