Our absolute can't live withouts were:
Lots of old school flat nappies (used for spill clothes, floor play mats, changing mat, makeshift bib etc)
Good bibs - I love the Mum2Mum range with the plastic backs that don't soak through to the clothes underneath.
Cloth breast pads
AIO's - DD lived in those for the first few months
A good wrap
Pram or carrier but if I did it again I'd be tempted to hire one or the other to start with and then go out and look for what suits your needs when you have baby. Both times we bought what we 'thought' we needed and both times we ended up going and buying a different pram/carrier which was kind of annoying! Getting to a Slingbabies meeting to try out different carriers would be a great idea if you can to see what's out there as there are so many options, either before or after having baby (or both would be best!)
Everything else in my eyes was a 'nice to have' but not necessity. DH was there to pop out and grab things as we needed them (ie. bigger nappies, nipple cream)
ETA: Of course a car seat as well
Edited by MrsEmma