First, thanks for starting the conversation, always appreciate developers who ask for feedback from their players.
You asked about favorites, I really enjoy Sir Finley's diamond card; it's unique, clever and eye catching. Some good runner ups - Loatheb is mysterious and conveys a foreboding mood and Ragnaros, the Firelord looks beautiful and mesmerizing. In terms of least favorite, of course my input only reflects my tastes but I think Blademaster Samuro is rather uninteresting - doesn't look dynamic, evocative or tells a story. Runner up for for least favorite, Jandice Barrow - she just sort of sways back and forth. My favorite diamond cards convey a moment in time and tell a story (Loatheb), make good use of the animation cycle (Sir Finley) and, of course, make good use of a color palette (Balinda).