I don't have one favorite; I've fallen head over heals for all members of the zygopetalum family. But if I had to choose three, they'd be (plants in the pics aren't my plants):
Paphiopedilum micranthum... if Paph rothschildianum is the king of Paphs, then micranthum is surely the queen. I fall for that big pouch every time. Interestingly, the mating of these two species creates one of the most striking slipper hybrids ever!