As seen on the picture, The Picture below shows a collection of Rare Dragon Eggs.

The Two rarest Dragons are offically said by the creators of DragonVale that Dark Rift and PlumeWing are the Rarest Dragons in the game.

As seen in the Picture Below, Dark Rift and Plumewing are not listed, as for they are very rare and not considered Basic. They still can be earned in few ways though.