What is the rarest and best gear to get for a dark knight

I'm talking about the most powerful rarest end game gear

I wouldn't call it rare, but the gear from Alexander is the best at the moment.

http://xivdb.com/?item/10388/Thavnairian-Bolero
http://xivdb.com/?item/10393/Thavnairian-Bustier
These are currently rarest

The best will always be from highest level raid, which is current Alexander right now, next week will have Alexander savage, and as more raids requiring the previous Alexander savage raid to access are added, the gear from there will become more rare.
Highest level tomestone gear was always equal to highest raid gear, but not rare

The rarest gear and the best gear are not necessarily the same.

Weapon-wise, the best comes from Ravana Extreme. In terms of rarity, however, I doubt many people have the Allagan, High Allagan, or Dreadwyrm greatswords, each of which is a 1/13 drop from final bosses of the three series of realm reborn raids, The Binding Coil of Bahamut. Given that these weapons were only added with the release of Heavensward, at which point that content became obsolete, I suspect very few people have made the effort to return and farm those drops. I also suspect several other weapon models that can be crafted, such as the Wootz Fausse, or the enhanced Primal weapons, are pretty rare as well due to a lack of people crafting them and potentially huge cost of materials.

Gear-wise, the best (and rarest owned due to being so new some pieces are literally not capable of being obtained yet) is almost certainly the Prototype Gordian gearset, which is obtained through the Alexander raid. The genuine Gordian gearset will be added in about a week with the Savage mode of this raid. This is discounting things like the Thavnarian Bolero for glamour purposes though which have no relation to class at all... they're just made with stuff that takes a pretty significant effort or wallet to obtain.