My opinion page addressing inquiries concerning clawhammer technique.
Either the index or middle finger will work well. I'd suggest you spend a week or so experimenting with each finger to see which one feels "right" to you. Go with whichever feels the most comfortable and allows you to keep your right hand relaxed while playing.
I, personally, use a slightly unorthodox approach (although I've encountered others who also do this). I use the middle to strike the first string and index for strings two, three and four. Very comfortable and distributes wear evenly between the two nails.
Thanks... Mike