Well, Reika's stuff for one. Give me a bit to look over my mod list and find the other couple.
Edit:
A decent portable crafting table (I use iBench).
Portal/Gravity Gun by iChun
Chunkloading with decent reporting. XU2 has chunkloaders yeah, but without chunk view and a loaded map... it's more trouble than it's worth.
Logistics Pipes
Decent compact storage (TE Caches/JABBA Barrels/Storage Drawers)
How are the TC addons for updating? I know witchery is on hold for right now. Blood Magic has updated, NEI's replacement is much better (though more handlers would be great). Aura Cascade, TE, Mekanism would all be nice...
But the bare minimum I could stand for very long would involve at least one major mod like Mekanism, EnderIO, or Botania. Reika's RotaryCraft and ChromatiCraft. and then a smattering of other tech/magic mods to keep the game interesting.