I've thrown together a few packs for myself when I want to check out the new Minecraft version and I'm not patient enough to wait for one, or even for all the usual mods to update, so I actually have a mental checklist for this sort of thing.
Specific essentials:
NEI, Iron Chests, Inventory Tweaks
General Essentials:
-Some sort of minimap (I get lost horribly without one XD )
-Item transport (whether pipes, or golems, or Railcraft)
-Auto-mining (turtles/arcane bore/quarry/other)
-Ore processing (once, all I had was TiCon)
-Sorting system
-Biome/terrain mods (to make the world more interesting because I like exploring)
-Mod(s) I've never played before and feel like trying out because they look interesting (sometimes found just by browsing the MC forums)