The best plan is going to depend a lot on what you need. We signed up DH this past year for self-only coverage because his dentist told him he needed a root canal and several crowns. For that kind of extensive work, we found that United Concordia offered the best coverage for the money.
With that said, now that everything has been completed, we're terminating the policy. Unless you anticipate a lot of cavities, braces, or the kind of really expensive stuff I referenced above, supplemental Federal dental insurance isn't worth the cost of the premiums. Get on a regular health plan (something like BCBS basic or maybe Kaiser) that covers cleanings and x-rays.