I have several plain white onesies, mostly with popper fastenings between the legs. Some pure cotton, some lycra. My favourite, though, is a PVC one that has no fastenings between the legs, just at the neckline. This works brilliantly as an anti-tampering cover for diapers, but I'm toying with the idea of getting some of these:
http://www.pjama.co.uk/
to deal with the occasional leaks from the more conventional diaper/plastic pants/ onesie attire.