If we are talking about the printing design or patterns on a onesie I don't really know, I don't think I could say I have a favourite, there are lots of cute designs out there and many many more cute fabrics that could be turned into a onesie.
If we are talking about design meaning the actual form and shape of a onesie though then I definitely have to say my own design is my favourite, you can see the idea behind them on my
Sewing Projects thread.
There is a solid reason for this. I find that if I'm sitting around at home not doing a lot a standard onesie is fine, but if I'm going out and walking around a lot (work, shopping etc.) then the location of the snaps in a regular onesie (right between the legs at the lowest point) rubs against my thighs and I end up with severe chaffing or rashes and at worse blisters from the snaps rubbing against and pinching at my legs.
I then found this onesie on Privatina. Which appeared to fasten higher up and closer to the waist band.
So having been learning to sew my own cloth diapers at the time I took a step further and made these.
I wasn't so keen on the side flaps on the Privatina ones so I did away with that, and worked on a way to have the flap come up from the back all the way to the waist and snap on there.
The design works and is wonderfully comfortable, since those pics were taken though it has been through some revisions to try and reduce the width on the from a little so it is more form fitting and less of the front of the thighs are covered, it still isn't quite perfect but getting there.