Although there have been many wonderful Transformation decks published for over 200 years, the Murphy Varnish deck ranks at the top of the best ever decks. Published in 1879 it features amazing, clever court cards, great transformed pip cards (the pips on the front of the card have been incorporated into the design of the card). It qualifies in the Hochman Encyclopedia of American Playing Cards as not only a Transformation deck but also as an advertising deck as there are advertising slogans for Murphy Varnish on almost every card. Perhaps there are members out there in Discourseland who can share other Transformation decks with us.
I still remember seeing a card from this deck on the cover of "Clear the Decks" not too long ago. I thought it was interesting, the many ways they advertised their product - it's practically invasive!
I have a favorite deck designer, Emmanuel Jose, who's had one semi-transformation deck printed with two more in the wings. You've probably heard of him and his decks - he uses giant images made of cut paper as art pieces, then has them copied and reduced to print them as playing cards.
The Curator Deck - into its second print run. No specific theme other than the medium itself.
Clipped Wings - coming soon to Kickstarter. Bird-themed. I had the honor of naming the deck for him!
Sawdust - third in the series, awaiting print. Circus-themed.