I always giggle whenever I spot the Inevitable Republic of Deseret, but that's not my favorite. Neither is any form of Michigander independence my favorite. No, I'd have to say my favorite is any form of an independant New England, where the America Revolution was isolated to Massachusetts, New York, Pennsylvania, and just the Dominion of New England and Middle Colonies area in general.
A unified Belgium and Netherlands always catches my fancy as well.