Pants with feet, pull on, with SEPARATE shirts. But I suspect that someone who isn't ECing will enjoy all those blasted footie pajamas. (Sorry, bit of a pet peeve there, we told dh's family that Lina can't wear footie pajamas because of night sweats (the same ones dh had as a kid) and the only clothes we ever get from them are those.) But pants with feet are sooo much better than socks.
As for most useful, a sling. Regardless of what someone ends up liking best if they get into babywearing, a pouch sling is really easy to learn and works really well with a newborn.