Metallurgy is usually where I start (teching up a grinder and furnace), then TiCo for tools, then Mekanism. My general goal is to have a steel pick/hammer, a shovel/excavator/hatchet/lumber axe (preferably platinum), a refined glowstone sword and a digital miner with max energy upgrades and whatever speed upgrades I can support power-wise by the end of my first couple days in a world.
At that point I start working on Ender IO for conduits, Big Reactors for power, and some form of farming.
I'm tempted to start with magic stuff in my next world, but Thaumcraft seems to play better if you've already built a bunch of other stuff that you can scan, Ars Magica I just don't know well enough to rely on starting up, and Blood Magic is a pain until you have some form of regen (and the spell system lacks in-game documentation and seems confusing...I love the rituals, the sigils, and the gear, but not the spell system.)
At that point I start working on Ender IO for conduits, Big Reactors for power, and some form of farming.
I'm tempted to start with magic stuff in my next world, but Thaumcraft seems to play better if you've already built a bunch of other stuff that you can scan, Ars Magica I just don't know well enough to rely on starting up, and Blood Magic is a pain until you have some form of regen (and the spell system lacks in-game documentation and seems confusing...I love the rituals, the sigils, and the gear, but not the spell system.)