Certified GKH Gardening Expert
There isn't really one "best" company to buy seed from, but you can narrow your decision with some knowledge of what you are looking for a a seed company.
All seed companies must guarantee a minimum germination rate and be certified that their seeds are disease free.
Beyond that, there are seed companies that specialize in heirloom, non-GMO, hybrids as well as different plant species.
For myself, I buy from Burpee, Baker's Creek, Tomatofest, and Seed Saver's Exchange. These are just a few of the high quality companies out there that you can buy seeds from.