Can't narrow it down to one, there are so many things that If I hadn't of had at that moment then I would have lost either the ewe or the lambs or even both.
1) lambing ropes
2) Spectrum
3) Combivit
4) Ketosaid (Had it in the box for years and never new what it was Until I needed it. If I hadn't of had it to give tot he ewe she would have died before the vets was open and for a bottle which is under £10 or a call out from the vet over £100)
5) Pen and strep
6) Twin lamb drench
7) Iodine

lube
those are just the very essentials but I do have more than this, I couldn't go into lambing season without a full box. I have shoulder length gloves but if you are clean then I find it easier with out gloves. Everybody will have their own individual make up of a lambing box though