EnderIO.
Started out as replacement for some TE items because that mod was not available yet. But it has grown so fast and to so many features that I use it now almost everywhere. Especially now that the power/fluid/item network can also contain the ME cabling.
Plus multi block energy storage, where you can upgrade the power input and output as the network grows. And some interesting machines, tesseracts and now also some form of teleport/travel?
Rotarycraft.
Unique, complex, challenging and lots of functionality. Simply brilliant.
Unfortunately the only of Reika's mods that I still use. Reactorcraft looks really interesting, but it adds so much fluff to the worldgen, that it got frustrating enough to remove the mod. (And Geostrata is even worse.) Which is a shame, because mods like BigReactors or Mekanism/Atomic Science show how it can be done a lot more elegantly.