Without a doubt, my favorite newborn sweater to knit and to put on V is puerperium. It is a great little cardigan, entirely seamless, which knits up in no time. I loved it so much, in fact, that I have made three so far (Ravelry details here, here, and here). These are so adorable on babies, and truly so easy to put on and take off of newborns.
I used Superwash yarns for all three sweaters, knowing that I would probably not be doing a whole lot of hand washing those early weeks. The cream and pink sweaters were knit using Cascade 220 Superwash, which felt a little scratchy at first but softened up beautifully after blocking. The gray puerperium was knit using Knit Picks Swish DK. That, too, softened up quite nicely, but felted slightly upon machine washing. It's the first time I have a problem with Swish felting, but at least it was only a tiny bit and nothing to extreme.
One thing I love about these sweaters is that they work equally well for a boy as they do for a girl. We didn't know we were having a girl, and so the first two sweaters I knit were done in gender-neutral colors: gray and cream. I also like that you could totally personalize the sweater by making it striped, adding an intarsia pattern, or even adding some texture to the body. And since they are so teeny tiny, you don't mind the stockinette body at all - by the time you are beginning to get bored of it, it's time to knit the bottom border and bind off!!