Both Frost and Unholy offer great and unique benefits and it is hard to say which is the better DPS spec. Frost will oftentimes offer more personal DPS and provides some buffs that, while not unique to only Death Knight, can be hard to get access to in a group without a Frost Death Knight; the same goes for Unholy as they offer a massive group debuff for all your spell casters in Ebon Plaguebringer. As such, it can be difficult to say which is the 'best' DPS spec, but both are strong and have a place in every raid. Try out both and see what you enjoy more as they both have fairly different playstyles.