Early game is probably the most fun for me. It's a greenfield and anything is possible. I try to focus on a different mod of playstyle each world, so I look forward to clicking 'Create New World' or starting a server without a world and watching it build from scratch.
But, if pressed, I'd say a selection of 3 chests, and you only get to open/choose 1. Once you open 1 chest, the others blow up, disappear, or turn to grass/dirt with flowers on top ;-)
The chests would roughly:
1. Movement/relocation chest
This is for people who prioritize base location over everything else. It would have a single-use (self-destruct) jetpack, single-use Boots of the Traveler or similar that take durability damage each block you step, and 2 MystCraft Intralinking (same dimension) books with 2 book stands.
2. Survival chest
This for people who hate hunting for wool, renewable food, and don't like building a dirt hovel. It would include wool, a crafting table, 24x-ish seeds, a shovel, hoe and bucket of water, a stack of cobblestone, 16x glass panes, and 2 wooden doors. It would also include a full set of leather armor and a bow with a stack of arrows.
3. Resource chest
Random resources - enough to build the 1st set of machines for (nearly) every ore processing mod with basic (Tier 1) power. Such chest might include, depending upon mods installed:
4x stacks of coal
1x stack each of cobblestone, sand, gravel, and Oak wood
32x each of clay and redstone
32x each of iron, copper, tin, lead, and silver ores
8x gold ore
Also, all chests would include a Thauminomicon if TC is installed. Not sure why we don't get one of those like how we get a TiCo starter book.