Arthurian Tarot [also known as the Hallowquest Deck]
This extremely beautiful deck uses imagery from the original tales of King Arthur, derived from the Mabinogion and other early sources and set into a realistic Romano-Celtic (rather than cod Mediaeval) context. The four suits are based around the four principal Hallows or holy artefacts of Romano-British tradition - the Grail, the Spear, the Sword and the Gwyddbwyll Board (a sort of mystical chess-board which represents the land).
The artwork is gorgeous and of a very high level of realism and accuracy - in fact the only flaw I could find in the entire deck is that The White Hart has a rounded behind like a horse's (deer come to a point at both ends).
On the down side, the meanings of the cards are often very different from those in a standard deck and in many cases - especially among the pip cards - the picture doesn't illustrate the meaning of the card at all clearly. Also, the cards are designed to have no reverse meanings: but rather than bowdlerizing the deck and using only the standard upright explanations, each card has a great long string of possible interpretations, both good and bad, which depend on context. As a result, it's an absolute pig to learn. I use this deck myself, and after 18 months of regular use I still sometimes have to look cards up in the book.