Michigan probably has one of the more unrecognizable borders because the state is usually defined by natural geological features rather than cartographic ones.
The hard ones for me are all those square-ish western states, that were carved out of one territory. Or maybe all those little NE ones clustered together.
Lousy-ana, FL, and TX are three of the easiest (of the continental states, of course. HI and AK are a cinch).