It depends on what you're making, where the baby lives, and the people you're giving it to.
Personally I prefer natural fibers for baby, so I would prefer wool or cotton over acrylic any day. For a baby in a cold winter area, wool is ideal since it keeps them warm at the same time as it breathes. Acrylics don't breathe, which can lead to overheating. Use superwash if you're concerned about washing. Knitpicks has some lovely soft superwash wools and they're pretty cheap, too.
Most manmade fibers also do not absorb, so anything that might get wet, from drool, leaky diaper, spit-up, should be an absorbent fiber, which would be pretty much your natural fibers, wool, cotton, even linen. I've never tried corn, soy or bamboo fibers, but I'd imagine they'd be more absorbent than acrylic.
If you're going to use a fiber like cotton, be sure you allow for shrinkage. Alternately wash it before knitting it up. It makes it a bit harder to work with, but you don't have to worry about as much shrinkage.
And any natural fiber other than regular wool or organic cotton needs to be washed several times before being put on a baby. It takes several washes for the sizing to be removed and the yarn to get to it's naturally soft state.