The best company is Family Tree DNA. They have the widest range of tests, the best customer service and the widest range of projects. There is already a very large pre-existing Miller project at Family Tree DNA. If you order a kit through a project you get a discount. If you would like to compare the services offered by the different testing companies then check out the ISOGG comparison charts:http://www.isogg.org/ydnachart.htmhttp://www.isogg.org/mtdnachart.htm
Bear in mind too that SNP testing is very important for haplogroup assignment. Companies that don't offer SNP testing sometimes get their haplogroup predictions wrong.