Envelope neck vests. When you get a massive sick or poo explosion you can pull the vest down to take it off, thus avoiding gunk in hair. You can get excellent quality packs from M&S for £7 for 7.
I much preferred babygros that did up down the front and between the legs. Anything that goes over the head is a pain for the above reason.
Babygros that do up down the side are a pain (I'm talking to you, Next). It's so hard to get the leg in the far side on a newborn that hates getting dressed and undressed.
Poppers for jackets and cardis are much better. Undoing soggy buttons on sicky cardigans is horrid.